Hi Tom,
Is
this the document you found?
The long and the short of it is, if you need to use custom entities, or really customize the system at all, you're going to need the Professional License. Basic and Essential are really for those Microsoft customers that plan to use CRM "out of the box."
Basic does allow some basic CRM access, but excludes SFA, Marketing Campaigns, etc. Typically, our customers seem to gravitate towards Professional licenses just because this gives them the ultimate flexibility to customize their system as needed and not
worry if certain sets of users won't have access to functionality.
Hope this helps.