I am wanting to use UDTT in my database, but we have CLR turned off, and according to the docs for create type, all types require a CLR assembly
User-Defined Table Types are a TSQL feature, not requiring CLR to be enabled.  Nor are they CLR types &quot;under the covers&quot;.  eg
exec sp_configure 'clr enabled', 0
reconfigure with override

go

create type tableType as table(id int)

go
declare @t tableType
insert into @t(id) values (1)

select * from @t

Then to show it's not a CLR type &quot;under the covers&quot;  run
select * from sys.dm_clr_appdomains
go
declare @t hierarchyid =  hierarchyid::Parse('/1/1/3/')
go
select * from sys.dm_clr_appdomains

to see how there's an AppDomain created when you create a hierarchyid but not a User-Defined Table Type.