Offclass
In 6v6, an offclass is a player of any class apart from the standard line-up of Scout, Soldier, Medic, and Demoman. These are situational classes, most often switched to by a Scout or Roamer when holding or pushing last in 5CP.
Heavy
The highest potential DPS (damage per second) class in the game. Heavy is best played in highly defensive situations. His lack of mobility, compounded with the fact that he is focused greatly once his presence is known, makes him difficult to utilize to great effect during offensive pushes. The best way to use him offensively requires great positioning so that the opposing team cannot run away. If a team is able to keep the enemies all in one place and in the Heavy's line of sight, said Heavy can be highly effective.
Places where he is best used include, but are not limited to, last points on 5 CP, defending on A/D maps, defending the point on KOTH maps, and pushing into certain last points. Additionally, having an Ubercharge is almost necessary when using a Heavy to fight. Once a fight has begun, there is no way for a Heavy to disengage. His low mobility and large size make him an easy target, contrary to Scouts, Soldiers, and Demomen, who all have the ability to escape in some capacity. Once he the Heavy is engaged in a fight, tanking him with the uber is almost a necessity. If played correctly, Heavy is one of the strongest offclasses.
Engineer
Similar in use to the Heavy, but even more situational. He is run almost every single round on defense of Gravel Pit, the primary A/D map, for his Sentry Gun, Dispenser, and Teleporter that can all be set up before the round starts in order to hold a point, usually point B. Being able to defend the buildings is crucial, and moving them as pushes begin is the only way to keep them from going down. If the Engineer works with his team, this can all be done smoothly.
On an average, more often-played 5 CP and KOTH maps, effectiveness of an Engineer is limited. The time it takes to set up even a single Sentry Gun with only one Engineer often offsets its benefits. It can be used to guard the last point, but it is much easier to destroy than a Heavy, and has no mobility unless moved (taking away all of its firepower). A team can sometimes also opt for an Engineer offclass if they know the enemy will be pushing with the Kritzkrieg, as crits do not affects damage on buildings, and the Kritzkrieg does not protect the affected players from incoming damage in any way.
The Engineer is the most situational of all the off-classes, and must be played at only the best times on the best maps.
Pyro
Once again, a class mostly used defensively. The Pyro has underwhelming damage, a very short range, and moderate mobility at best. His utility and usefulness come almost entirely from his airblast mechanic. He can be used in certain situations to flank and deal damage, but this ability is considerably weaker than a Scout's.
Because of this, a Pyro is usually run only when defending a last point on 5-CP. The airblast is used to deflect projectiles and hold an uber push in place. This is most useful when at an uber disadvantage. While the enemy team ubers in, the Pyro moves forward to airblast the combo backwards, stalling the push. The player may often die in this scenario, but it can be enough to delay the majority of their invulnerability, evening out the playing field considerably. Pyro isn't weak in and of itself; the other classes are simply stronger. If caught alone, a Pyro can easily be killed by a Scout or Soldier, making positioning very important.
Sniper
One of the most commonly used utility classes. The Sniper's very long range and very high damage can be used to pick off important classes, lock down specific areas, and deny jumping, rushing and suiciding players with ease. Positioning is the most important thing on Sniper, besides pure aiming ability.
The Sniper has base movement speed, 125 HP, and poor close range capabilities. If caught out by any other combat class, the Sniper will usually die. The best places to stand are those in which there is as long a sight line as possible towards wherever it is that the enemy will be coming out of, and somewhere where the Sniper can either escape very quickly or can be protected by teammates. A Sniper is not usually run at all times, so when switching to it, there is almost zero threat of being countersniped for the first life. This allows the player to take certain actions and positioning decisions that would be harder if there were a long range threat.
A Sniper can be run in many situations depending on the place the team holding on the map and the uber status. Snipers are most commonly run defending against an enemy with uber advantage, during an equal-uber stalemate, or defending/attacking a point with a particularly open sightline. There is no damage spread in competitive play, so a fully charged bodyshot will instantly kill a Medic and an unbuffed Scout. The primary target in 99% of situations will be the Medic. After that, there are few targets worth holding a shot for. The Demoman is the next most important, but wasting other potential shots and kills is not worth it in most situations. Knowing what maps to snipe on and when the best time to switch can win games for the team, so long as someone has the aim to backup the decision making.
Spy
Similar to the Sniper in his single-target killing power, the Spy is a highly situational "pick class" that excels during stalemates. The Spy lacks a lot of the utility that the Sniper provides, but can be more effective at times where a Sniper has little vision or killing capabilities. The Spy also has excellent scouting power through use of the Cloak and Dagger item.
Choosing Spy at the right time is the most important part of the class. Once played him once in a game, the enemy will be much more careful for the rest of that game. The first stab is the most effective. In an ideal situation, the Spy won't even be seen by the enemies until an enemy player is killed. Because of the high levels of communication and low amount of players in a game, disguises are very unreliable and will not fool most players. Additionally, it is unwise to disguise as anything other than the main 6v6 classes (Scout, Soldier, Medic, Demo). Disguising as Soldiers and Demos will cripple the Spy's movement speed, and a team usually knows where their Medic is, and it's very suspicious to see a Medic but not have him healing anything, so generally, Scout is the best disguise, even though the Spy is slower, this can be disguised with spamming jump and crouch, or just sneaking around with cloak and the small body of the Scout.
Due to all of this, the best watch to take when going for kills is the standard Invisibility Watch. It will get the Spy behind them the furthest and fastest. When trying to scout out the enemy position, attempting a back cap, however doing anything that requires more than 15 or so seconds of invisibility, the Cloak and Dagger is best. This can be used to invade the last point on a map and simply watch, or wait for them to push out and attempt a cap. Going spy at the right time in a game is the primary deciding factor in its usefulness. The Dead Ringer can be used while holding last to fake a death with friendly disguises and instigate an enemy push, but can be detected by listening to the Spy's death cry, or if they have ragdolls on, as the Spy's corpse is dropped, not the disguise.