See the spine-c API:
https://github.com/EsotericSoftware/spi ... tion.h#L48
"apply" means to set the skeleton pose based on an animation. This ignores the current pose and overwrites it with the pose from the animation. "mix" means to set the skeleton pose somewhere between the current pose and the animation pose, based on the alpha parameter. It is used most often for crossfading when animations change, so the change is smooth.
You can clear your animation whenever you want. This means no animation will be applied, so the skeleton pose won't change.
Documentation will be coming. Would you rather have documentation and no runtime, or an undocumented runtime?