Hi Hilary,
Here is validation rule:
current()/../my:VxGL != xdxDocument:GetDOM("VISA GL# Validation")/dfs:myFields/dfs:datafields/d:SharePointListItem_RW/d:GL_x0023_[.=current()/../my:VxGL]
"VxGL" is value entered in combo box.
"VISA GL# Validation" is secondary data source - I am trying to filter this using the VxGL.
Thanks and much appreciated.