It's the Cartesian Join operator. It matches any record in the first table to all records in the second.
Where can I find definitions of all the operators (relational and mathematic) in FMP? I've searched in Help and in the forum but have come up empty, so far. For instance, Help just spins and spins on "Cartesian" and when I search the forum for "operator definitions" I get zero results.
Nevermind, just found it in the Users Guide. I shoulda looked there first. Instead of looking in the Advanced Features Guide.