The Pocket is the player that the team's Medic focuses in and follows for most of the match. In 6v6, the pocket is almost always a Soldier, but in Highlander it can be the Soldier, Demoman, or Heavy.
Main Roles
The three main goals of the pocket are as follows:
1 - Protecting the medic
2 - Lead pushes
3 - Effectively use ubers
There are a lot of other roles the Pocket has to fullfil at some point, these are just the main ones.
The 'combo'
The Pocket is a very important part of the combo, as it is an effective bodyguard for the Demoman and Medic. The Pocket's job is also to watch main choke points with the combo, and to punish any enemies who overextend near choke points.
Mid Fights
The Pocket, being one of the most polyvalent 6s class, has a number of ways to approach midfights. Usually, this involves protecting the medic from bombing soldiers and scouts, while putting out consistent damage on the other team. In some cases it is known for both Soldiers on a team to bomb the opposing Medic in a mid fight rather than staying to protect their own. The Medic will often be left to the Demo during this period, however this is largely a team preference as they may be using a strategy that requires higher aggression than normal.
Pocket Unlocks 6's
The Pocket, being heavily attached to the Medic, will usually use a rather stock like loadout due to the lesser need of the Gunboats unlock. This loadout would commonly include A Rocket Launcher or The Original/Shotgun/The Escape Plan.
The use of the Original/Rocket Launcher is a common usage by all soldiers due to it's reliability of 4 shots which is capable of killing almost all targets when hit for rather large amounts of damage, the Heavy is largely excluded from this due to the ability of 450HP through overheal. The Solider on occasions may run The Direct Hit this is often to spam out a Sentry nest locations on maps such as cp_gravelpit as it is an effective use for the unlock with the lower splash and higher damage. The Direct Hit also may be used due to preference of the Soldier but this is rarely seen.
The use of the Shotgun on a Pocket is often used simply because the Gunboats are not required. The shotgun is a very well rounded weapon for a pocket to use due to a good damage output and 6 shells that can be loaded making it effective at taking out many targets both alone or after using rockets. The shotgun is also very good at helping to defend the medic as the role of Pocket as it will cause jumpers/bombers of the opposing team to be slowed if hit while in the air and is a safer alternative to airshots with rockets.
The use of the Escape Plan rather than a stock Shovel will often be seen due to Rollouts as the Soldier can cause enough damage to themselves easier than a Solider with gunboats allowing them to be granted the movement speed bonus and reach the midfight along side the rest of the team members easier. After 10 seconds this Soldier would then be able to take "Crit heals" from their Medic and be restored to full health upon arriving at the midfight.
This loadout is well rounded and allows a Pocket to protect the Medic in a number of ways, give out damage in a number of ways and also a possible escape should they become to injured to stay in a fight.