Answer variables are predefined shortcuts for entering information into an answer instead of a long string of text. When an answer displays on the customer portal, the variable is replaced with whatever value was specified by your administrator.
Variables are also helpful to use for information that may change. When your administrator changes the value of the variable, all answers containing that variable will also change. Contact your administrator for a list of variables defined for your application. For example, if your administrator added a variable for a support email address at your organization called "contact," you could insert the variable for the email address in the answer, such as $contact. If the email address changes, all answers containing the contact variable are updated.
Reference:
https://docs.oracle.com/cloud/november2016/servicecs_gs/FAABU/_BUIGuide-9.htm

20MB is the maximum as well as the default.
Reference:
http://communities.rightnow.com/posts/7e961ac4d6?commentId=46204#46204

Before publishing an answer, determine which interfaces and customers you want to be able to view the answer. You can control visibility with answer access levels, languages, products, categories, and conditional sections.
A conditional section is a section of text in a document that appears only to those contacts meeting certain criteria. Conditional sections can also be placed in answers and tagged with specific answer access levels. In message templates, conditional sections can be used to filter content by contact, profile, runtime variable, or record type.
Reference:
https://docs.oracle.com/cloud/november2016/servicecs_gs/FAABU/BUIglossary.htm#FAABUcondition

The thesaurus.txt file contains a list of words and their synonyms used in the Similar Phrases search technique on the Answers page. Words in the text search are matched with synonyms to extend the searching capability to include ontologies.

Knowledge Advanced uses Products and Categories hierarchies that you define for your Service Cloud environment, and automatically synchronizes any changes you make to the Product hierarchy.
You manage users through roles, which control the level of management access you have to Knowledge Advanced, the general authoring abilities you have on a particular Content Type, and what tasks you can perform in a workflow. Users are assigned either Console roles or Web roles.
Reference:
Oracle Service Cloud, Using Knowledge Advanced, Release May 2016, pages 28, 45