QCAD.org Forum

It's been a while since I see those acronyms on the bug tracker.
As I -kind of- understood while reading through posts in these forums, it seems these features are present in Autocad.
Does anyone please exlains what those stand for, and briefly what it is exactly? Thank you in advance!

UCS = user coordinate system, a local, user defined coordinate system to usually used for a part of whole drawing.
XREF = external references, blocks that are defined in external files, not in the drawing self.