Here is the quote from Standard license itself:
a. License Tiers. Content may be offered at different license tiers, and before completing a Transaction you may be required to select between a Personal - Reference Only tier, a Personal tier, and a Professional tier (each a “License Tier”).
So they are indeed called “license tiers” in fact ‘Personal’ and ‘Professional’ being the price tiers 
but ‘Reference only’ is really a license tier with more restrictions.
Exemplar guide from Epic: How to make simple licensing super complicated and lose customers’ trust.