What is an "operational master" with respect to entity-relationship diagrams?
You piqued my curiosity here. Cursory search on the internets mostly shows that this question haunts students since at least 2014, with no apparent relief. However, there seems to be one page that  explains what an "operational master entity" is:


  The key-only parent entity (otherwise known as an operational master entity, a collection entity, or a categorising entity, or perhaps a domain entity) stores the valid or actual range of values for the attribute.


A domain entity commonly translates to a dictionary, lookup, or dimension table. For example, for a Game entity a Publisher might be one operational master entity. The Publisher entity would have two attributes, a surrogate Id and Name.

P.S. It appears Google Books have a few more occurrences, mostly from the last century.