I felt this was worth blogging as it has been a subject I've seen pop up lately. You may have seen an animator talk about priority and scratched your head in confusion. The priority number (1 - 4) is chosen when an animation is uploaded.
When I create a pose it is made with the intention of just that, posing. Poses are uploaded with a priority of "4" which means that when in use it will override any other animation or pose with a lower priority number. Besides having a high priority number, I also pose every single body part of the avatar, making the pose ideal for photographs or modeling when the avatar needs to remain perfectly frozen in position (no typing or head movement). Due to this "frozen" state I never recommend my poses for animation overrides. Sometimes there are exceptions, for instance when an override is used for a purse or prop it may become necessary to prevent typing or head movement. As I said this is just a recommendation, a person can certainly use poses in their override if they choose to, they just need to be aware that it may interfere with other animations.
Whenever possible, I will create an animation override (ao) set from a set of poses I've created. I will only use the poses that work with head movement and typing. I generally call these "stands" and they are uploaded with a priority of "2". Using a priority number of 2 allows the typing animation while still holding the position of the stand and I remove any posing of the head so that it will move freely. Gestures, dances, etc, which are usually high priority (4), will override the stand. Low priority stands (or ao stands) can still be used on the runway of course, there are no rules set in stone lol.
I must mention that regardless of priority number, if an animator has not actually posed a body part in the program they use (I use Poser), it will move in world. Have you ever noticed with a particular stand or pose that sometimes your arm moves oddly? This is often because the animator did not pose some part of the arm (collar, shoulder, or forearm) and it is still being effected by the default stand cycle.
I've only mentioned poses where the avatar is standing thus far but priority comes into play for all animations. Hopefully this little bit of reading hasn't confused anyone further lol. There is so much more information I could dive into but I will save you the eye strain, for now :P.