See also: Notation#Stances
Stances are temporary if they lapse on their own or stable if they don't. Stable stances are more often considered “real” stances.
A stance might have a manual transition (i.e. one with no other move attached), optional transition (done with a specific input after certain moves), or mandatory transition (happens automatically when doing certain moves). It could also be forced by an opponent, such as being forced into crouch or back turned.
Generally, stances give the player access to better mixups at the cost of some defensive ability or predictability. For example, Leroy's Hermit Stance (HMT) has a strong plus-on-block mid and two strong lows, but he can't guard, backdash, or step block.
However, some stances, such as Lee's Hitman Stance (HMS) or Eddy's Negativa (RLX) are mainly used for their evasive properties. Their weird hurtboxes make it difficult to start pressure after blocking anything that can transition to them, because the counter-attacks can whiff easily. The most infamous of these is Xiaoyu's Art of Phoenix (AOP), whose manual transition can evade mids almost instantly.