What object types should be in schemas?

The current hierarchy of object types is like this:

database
    access method
    event trigger
    extension
    foreign data wrapper
    foreign server
    language
    publication
    schema
        aggregate
        collation
        conversion
        domain
        function/procedure
        index
        operator
        operator class
        operator family
        sequence
        statistics
        table/view
            policy
            rule
            trigger
        text search configuration
        text search dictionary
        text search parser
        text search template
        type
    subscription
role
tablespace

special:
- cast
- transform
- user mapping


How does one decide whether something should be in a schema or not?  The 
current state feels intuitively correct, but I can't determine any firm 
way to decide.

Over in the column encryption thread, the patch proposes to add various 
key types as new object types.  For simplicity, I just stuck them 
directly under database, but I don't know whether that is correct.

Thoughts?