Since engine version: 1.0 OC
The value depends on the phase of the current action. The phase is mapped linearily to the length of the animation the current action is playing. This can be used to play animations synchronized to the ActMap animation.
See the animation documentation for further explanations of the animation system.
PlayAnimation("Walk", 10, Anim_Action(), Anim_Const(1000));
Plays the animation "Walk in slot 10 synchroned to the current action.