4D makes a duplicate of the selection address table when you copy the current selection for a table into a named selection. The original current selection still exists along with the new named selection. In client/server the new named selection exists on the server. It requires 4 bytes per record plus an additional 4 bytes. Unlike the current selection the named selection must fit entirely into memory (the current selection may be partly stored in temporary files on disk). 4D will not create the named selection if there is insufficient memory to hold the named selection. 4D will generate an internal error - the user however will not see an onscreen error message. You will need to trap for this error with ON ERR CALL.
Note: This command may be slow with large selections.