There are two Limit Destroyer entries in the Q-store
One which look like a module : https://q.subliminalclub.com/product/limit-destroyer/
One which is an Ultima core : https://q.subliminalclub.com/product/limit-destroyer-ultima-core/
I guess both Limit Destroyer are the same and are a core however this is misleading but it would be better to have it clarified.
@RVconsultant @DarkPhilosopher or anyone?
I think the original idea was to use the Q version when building a Q Custom, and the Ultima Core for an Ultima Custom.
Some such Ultima-Core-double-titles may have been removed later though, because the system auto-rewrites Ultima scripts for any Module now – when the Build is selected.
Please submit a support ticket for an official answer, then please post the answer here.
Before Ultima, there were Superchargers.
The Q Store came out in late spring/early summer last year, while Ultima came out late last year. Ultima cores showed up early this year. With the supercharger versions of Limit destroyer, Rebirth, Sanguine, Godlike Masculinity in the Q store, you can make customs like below 
while staying within the 4 core limit.