qa_userevents table contain all event information aobut the user such as question, answer, comment etc..
Refering to the table
entity type will store the entity type
Question as Q, User as U, Tag as T, Category as C, If none as -
While updatetype column will store some kind of event information for instance update category for the question, question closed, content edited etc.
You can find more details in qa-app-updates.php file