"Maximum operating level (MOL): The specified maximum water level under normal operating conditions. The MOL shall be taken as he MWL unless otherwise specified.
"Maximum water level (MWL): The water level defined by the lip of the overflow or weir unless otherwise specified."
These definitions are from AWWA D100-21 (welded tank standard), but I assume are the same as D103. This is in Section 1.2, Definitions, in D100-21.