What is an "operational master" with respect to entity-relationship diagrams? I have a college assignment to create an entity-relationship diagram (ERD) for a database we're going to build. That's fine, but I have to include something called an "Operational Master" in the ERD. The lecturer's description is vague at best, and I can't seem to find any info about it online.

Basically speaking, we have to create an ERD for a game rental service with at least one "operational master". The only other reference to it in the lecture notes was to "add an index for the 'operational master'". It was never really explained during the lecture as far as I can recall. I get what an index is, but not what is meant by the "operational master".

Has anyone any ideas on what this might be?
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.