Books with specialisations?

Hi all, I'm taking a look through the Mythic Locations book and I've noticed the following Tractatus on page 91, listed as follows:

Athenaeus, Edicts of Augustus (Tractatus, Civil and Canon Law (emperors), Quality 8)

What does the (emperors) mean? Is it a specialisation? I'm only familiar with the Core Rules for 5E, which I believe give you a specialisation for each Ability, which gives you a +1 when using that skill. How does this interact with a book having a specialisation? Do you get a +1 when your specialisation for the ability matches up with the specialisation of the book? Or do you gain that specialisation for the relevant ability in addition to your previous specialisation perhaps?

Is this detailed in another supplement I haven't read? Thanks