User blog comment:Mh314159/Help me understand a natural number recursion/@comment-35470197-20191014142325/@comment-39541634-20191016111747

It is known that TREE is somewhere between the SVO and φ(ω@ω) in the FGH (the second bound is due to the fact that the termination of TREE(n) can be proven in a certain theory with a PTO of φ(ω@ω)).

So saying that someting has a growth rate roughly comparable to TREE actually makes sense.

(as a side note, in my letter notation the first letter that outgrows tree is "T". Easy to remember: T for TREE :-))