Where is the ‘abaft the beam’?

Here is a another picture which points out all locations on a boat and which indicates where 'abaft the beam' refers to, whether port or starboard beam.

I have highlighted the relevant terms for you, in red.

'Abaft' is formed (from 'baft') within English by derivation, says the OED

'Baft' : 1. Of place: Behind, in the rear; in later usage only Nautical: Astern, aft, abaft. arch.

As I understand it, just looking up the nautical descriptive terms, "abaft the beam" would apparently be just behind (toward the rear of) the widest part of the ship at center. From your picture diagram that would represent the square outlined area just behind the middle of the boat.

"Abaft" generally means "behind or toward the rear" and the "beam" of a boat is generally the "width at the widest place". From what I gather, if "starboard" or "port" is also used describing the "beam" position, that means to about 45 degrees behind the widest part of the ship on the right side or on the left side respectively.

https://www.thefreedictionary.com/abaft

https://encyclopedia.thefreedictionary.com/Beam+(nautical)

https://people.well.com/user/bkoball/Compass_Points.pdf