The functional difference would be: A program with implementation-defined semantics is always well-formed and the behavior of the implementation must be documented.

From: SG5 <sg5-bounces_at_[hidden]> On Behalf Of Victor Luchangco via SG5
Sent: Monday, September 28, 2020 6:54 PM
To: sg5_at_[hidden]
Cc: Victor Luchangco <victor.luchangco.work_at_[hidden]>
Subject: Re: [SG5] Points raised on EWG reflector

What is the functional difference between "undefined behavior" (I assume this is what UB stands for) and "implementation defined"?

Presumably, a particular implementation can implement "undefined behavior" however it likes, and in particular, it can do so in a predictable
and understandable way. Does saying that behavior is implementation-defined create an onus on implementors to actually give well-defined
semantics to a feature? (I agree that we do not want to create any such onus.)