What is used to determine user access to knowledge bases or a knowledge article?

sn_kb_read and sn_article_read are typically roles used for knowledge base access, but User Criteria is the mechanism that utilizes these roles (among other conditions) to grant access, so User Criteria is the broader and more accurate answer.

"Privacy Settings" is too general and not the specific ServiceNow feature used for knowledge access control.

A "Read Access Flag" is not the specific ServiceNow feature for controlling knowledge access; User Criteria provides the detailed control.

User Criteria are powerful conditions used in ServiceNow to control access to knowledge bases and individual knowledge articles based on roles, groups, companies, locations, and other user attributes. They provide a flexible and granular way to manage who can read, create, or contribute to knowledge.