Jim Kanter’s explanation is correct:
On Mac use the OPTION key with the property shortcut key.
On Win use the ALT key with the property shortcut key.
A Anchor Point
P Position
R Rotation
S Scale
T Opacity
For example, if you’re setting a keyframe for scale and you’re on a Mac press option + the S key, if you’re setting a keyframe for opacity on a Windows machine press Alt + the T key, etc.
I was confused too but just happened to figure it out. Hope this helps!
-Erik
Video Beast