The Soldier is a class that can do damage at both close and long ranges. Typically, a standard 6v6 lineup will include 2 Soldiers; 1 roamer and 1 pocket. The pocket Soldier tends to protect the team's Medic while the roaming Soldier watches the flanks and attempts to help in fights or go for strategic kills.
Main article: Pocket
As a pocket Soldier, your primary job is to keep your team's Medic alive. The pocket also has roles to fulfill in Mumble (or other communication program), because, in some teams, he is going to be the 'Main Caller. Generally, the pocket Soldier leads his team into battles; he soaks up damage while putting out his own damage. Most pocket Soldiers equip the Shotgun for the extra damage output to defend against enemy players. Remember, your job is to protect the Medic, as his life is worth more than yours is. If saving your Medic means sacrificing yourself, you should do it. To be a good pocket, you also need to have a very good understanding of 6v6, something the community commonly calls 'game sense'.
Main article: Roamer
The roaming Soldier typically has the stock Rocket Launcher or the Original, Gunboats and Escape Plan equipped. Of course, there are different loadouts being used, but this one is the most popular one by far. The roamer has different roles to fulfill in 6v6. The first and most basic one is to create distractions for his team so that they can push or take an advantage of it. His second role is to be on the flank with the Scouts during stalemates to try to get a pick, or simply to just block the enemy flank. The Roamer's third role is quite similar to the first one. When his team is at a uber disadvantage, or simply during a stalemate, the roamer will often be buffed to 300 health and sent in to try to get the medic to pop his uber, or to kill the medic. In certain situations, the roamer may offclass to try to gain an advantage. Another role is that on mids, the roamer has to reach it quickly and efficiently. Each map has different ways to rollout to mid and they all have their positives and negatives.
There are also a handful of video and text guides and tutorials created by the community on the soldier class.
- Fragga Soldier Tutorial - Fragga at his prime was considered one of the best soldiers to grace the game
Rollouts.tf has a collection of rollouts for all of the 6s classes.