From "help"
visretain set to 0:
The layer table, as stored in the reference drawing (xref), takes precedence. Changes made to xref-dependent layers in the current drawing are valid in the current session only and are not saved with the drawing. When the current drawing is reopened, the layer table is reloaded from the reference drawing and the current drawing reflects those settings. The layer settings affected are On, Off, Freeze, Thaw, Color, Ltype, LWeight, and PStyle (if PSTYLEPOLICY is set to 0).
visretain set to 1
Xref-dependent layer changes made in the current drawing take precedence. Layer settings are saved with the current drawing’s layer table and persist from session to session.
the bottom line . . .
If you set visretain to "0" in your working drawing, when you reload the xref or open the working drawing, it will display all of the xref layers in their last saved state in the source drawing. Any changes you made to them in your working drawing will have been lost.
If you set visretain to "1" in your working drawing, when you reload the xref or open the working drawing, it will display all of the xref layers in their last saved state in the working drawing. Any changes you made to them in your working will be preserved. Any new xref layers will be displayed their last saved state in the source drawing.