Changing one thing
On a space, tap a point or paint over the part you want to change, then say what should change. Revision Lock keeps every pixel outside what you painted exactly as it was: the camera, the size and the rest of the picture stay put.
Changing the light is different: light falls on the whole picture, so nothing is held still there. The screen says so before you press.
Versions
Every change is saved as a new version with a short summary of what changed. You can go back to any version; restoring one makes a new version, so nothing is lost.
Before and After lets you compare a change with the picture it came from.
When a change goes against your brief
If what you ask for conflicts with your brief or plan, Studio says so and still makes the change. You decide.