Related to sounds, visual effects, UI-related features, etc.
Affects gameplay balance.
VM aborts, crashes, or other critical bugs.
Something doesn't work the way it's supposed to.
Comments, manuals, etc.
Related to features, be it adding, removing, or improving new features.
Requires major codebase changes to accomodate the change.
Makes things go faster.
Changes aimed at improving quality of life when using a feature.
Under-the-hood codebase improvements.
Will result in progress loss or something may break.