REFPATHTYPE

Type: integer · Stored in: session · Initial value: 1

How the path of the external references and PDF underlays you attach is saved.

Value	Path
0	No path: just the file name, which is looked for in the drawing's folder
1	Relative to the drawing's folder
2	Full

XATTACH offers it in its pathType option. PDFATTACH saves a relative path with 1 and a full one with 0 or 2.
If the drawing has not been saved yet or the file is on another drive, the relative path is saved as a full one.
It applies to the session: it is not saved in the drawing (in AutoCAD it is a program setting).