Which type of tables may be extended by other tables, but do not extend another table?

A base table serves as the top-level parent in a table hierarchy, meaning other tables can extend its functionality and fields, inheriting its properties. However, a base table itself does not extend from any other table, making it the foundational table in its specific hierarchy.

"Core Tables" is a general term for fundamental ServiceNow tables; while many core tables are base tables, the term "Base Table" specifically defines the extension behavior.

Extended tables *do* extend another table, which is the opposite of the behavior described in the question.

Custom tables can be either base tables or extended tables, so "Custom Tables" is not exclusively defined by the behavior of not extending another table.