Idle is both a status and an animation. Essentially if an entity isn’t performing some specific action, it’s probably idle.
Animations
There are several types of animations an entity might assume while it is idle.
Idle
IDLE is the most basic animation available, and in most cases a model isn’t functional without one. Player characters and AI usually play IDLE when standing still. Most projectiles are playing IDLE after they are thrown or tossed. Obstacles, traps, panel entities, and most other types are also playing their IDLE animation by default.
Normally there is only one Idle, but you may add additional Idle# animations (Idle2, Idle3, etc.). These animations override Idle when hostile entities are within their range. As an example, you might want your character to assume a ready stance when enemies are around, and relax when alone. To do this, you would place the relaxed pose into Idle and then define Idle2 with a range setting that covers most of the screen. When enemies are in sight, your character will take up their battle stance.
Sleep
If available, this animation plays after staying in IDLE# for 10 seconds. You can modify the amount of time before SLEEP plays with the sleepwait property. If this animation has a loop, it will play continuously until you take further action. Otherwise the entity returns to IDLE#.
Faint
If available, FAINT overrides all IDLE# and SLEEP# animations when entity is at or below 25% of its maximum hit points.
Properties
Idle (State)
Idle is an internal state not user accessible outside of script (see entity properties). Generally, entities are considered to be in Idle state while walking, Jumping, or standing (IDLE#). By default, entities must be in Idle state before they will respond to player commands or further AI actions.
Sleep Wait
sleepwait <int>
#default
sleepwait 10
Model property. Controls number of seconds in IDLE# animation before entity plays SLEEP.