How can "HierarchyID" type work when "CLR" is disabled? The following statement sp_configure 'clr enabled' returns:

name        minimum maximum config_value    run_value
clr enabled 0       1       0               0


I assume this means there are no CLR objects; however if I execute the following statements I get these:

SELECT * FROM sys.assembly_files




SELECT * FROM sys.assembly_types




I have read that The HierarchyID type is available to CLR clients as the SqlHierarchyId data type..

How can it be used without having CLR enabled and why do I need to enable it to create my own CLR objects?
The clr enabled server configuration option only controls whether user assemblies can be run by the SQL Server instance.

The hierarchyid, geometry and geography types are system CLR types. These are contained in system assemblies, so are available regardless of the clr enabled setting. 

Similarly, other system features that rely on CLR integration, like FORMAT (SQL Server 2012 onward) are available even if clr enabled is off.

If you need to manipulate hierarchyid data in a custom CLR routine, you will need to enable user clr assemblies via this setting.

See also: Hierarchical Data (SQL Server)