"Throughput accounting" doubt (Clarification on a particular phrase)

A very good afternoon to you sir,
i need help understanding a phrase from the throughput accounting lecture notes.
Before going further i want you to know that ive spent about 8 hours watching and understanding your lecture word to word.

In the lecture notes, section 4 of throughput accounting, under definitions, this formula is given.

Throughput accounting p.m.h.. or Returns p.f.h.. = Throughput accounting/Time on key resources.

“Time on key resources” i don’t understand that particular phrase.
Please explain as to what it means and more importantly what it refers to in particular.

Also, go through this formula list of mine [ https://i.imgur.com/MQuMKY3.jpg ] and verify if each of the formulas are correct sir.

Thank you.