Session entity types are referred to as User entity types and are entities that are built for an individual user such as favorites, preferences, playlists, and so on.
You can redefine a session entity type at the session level to extend or replace a custom entity type at the user session level (we refer to the entity types defined at the agent level as "custom entity types").
Note: session entity types apply to all queries, regardless of the language.
Required. The unique identifier of the session entity type. Format: projects/<Project ID>/locations/<Location
ID>/agents/<Agent ID>/sessions/<Session ID>/entityTypes/<Entity Type
ID> or projects/<Project ID>/locations/<Location ID>/agents/<Agent
ID>/environments/<Environment ID>/sessions/<Session ID>/entityTypes/<Entity
Type ID>. If Environment ID is not specified, we assume default 'draft' environment.