Why not reference frame
? Isn't that exactly what they are?
a) reference frame, as in physics, does not define a coordinate system. In this case, we are talking of a container that does define a coordinate system.
b) in our case, we are talking that geometric objects belong to a coordinate system, and it requires additional effort to use the object from within another coordinate system (transformation between them needs to be taken into account).
Workspace seems to do a better job, because I can say:
"Object1 is in Workspace1"
"Objects within a workspace can link freely to each other", e.g. "I can Part Fuse a sphere and a cube because they are both in Workspace1"
"Objects in different workspaces cannot simply link to each other, they must use special link type and optionally account for coordinate system difference"
"Placement defines a coordinate system" (I mean, placement alone by itself, App.Placement)
"Coordinate system doesn't define placement, since coordinate system can be in general left handed, skewed and otherwise distorted"
"Placement doesn't define a workspace. Workspace is defined by a moveable container (e.g. PartDesign Part). Objects in non-moveable containers (like Group) within Part are in the same workspace, Part."