Typically used with: Endurance.
In short: tough it out. The Stamina skill is used when a character is performing strenuous activity. Many environmental effects require a Stamina check to shrug off, such as extreme heat, cold, thin air, or no air. Performing strenuous activity for longer than usual will also require one. Survivalists, backpackers, and free-divers would make heavy use of the Stamina skill.
Any time you're trying to push your body past its natural endurance limits will call for a Stamina check. In general, if you're trying to avoid moving down the Knockout Track, increased Stamina is your best defense.
The DL for environmental effects is varied. The more extreme the climate, the higher the DL. If a character is suffering from environmental effects (heat, cold, etc.) and fails a check, he moves down one step on the Knockout Track. A new roll must be made each hour a character is in harsh conditions.
The Adventures chapter details the use of Stamina to march, jog, or ride for long periods of time.
In an environment with no air, a character who is not doing anything physically taxing can hold her breath one minute for each point of Endurance she possesses (thirty seconds per point if performing strenuous activity such as combat or swimming). After this, the character is considered to be suffocating. For each round spent suffocating, a Stamina roll against a DL of 15 must be made in order to keep from slipping one step down the Knockout Track. Each round the difficulty increases by one point (DL 16, DL 17, and so on). Once the character reaches step five (unconscious), if the airless conditions persist another round, the character suffocates. One skilled in Healing may be able to revive a drowned character if quick to action.
Stamina checks are used against some Magic Spells and similar effects.