The Big M is to show the class of service you are in: Economy Class.
(Also called Monkey class

I remember when they had different colour Boarding passes:
Green for Eco
Blue for Business
Red for First.

Then they changed it to the same Boarding pass.
They got complaints because the first class pax could not show that
they have a cool red boardnig pass.

So they write the class of Service M,C or F in a big letter on your BP,
that you can show everybody what class you travel.

Recently they introduced the red Borading passes again.

Zones:
If you travel in a very high booked Fligth, they want to fill the plane as fast as possible: Window-Seats first, then middle then row-seats.
So every BP has a Zone.
Sometimes they call: Please Borad Zone X!
They do it not very often, it is not controlled, and it is as crowded as if they let everybody in as they want.

Hope that answers your Questions!

Best regards
Alex