Documentation for correct toolbar button for spells

[Firesong]

Hi all,

Hekili has the ability to tell you what button a spell is on but I'm baffled how they do it. I've tried breaking their addon a few times but the stack traces in BugSack make my head hurt.

Is there any documentation on this particular part of the WoW Lua API someone to which could provide a link?

Thanks in advance.