MAX_ENTS simply tells how many entities can possibly be in the map. So instead of writing "900 + MAX_PLAYERS * 15" all the time, you use MAX_ENTS.
!! means "no" to "no", but giving you a bool value (1 or 0). If you had number 10, it will return number 1, if you had 0, you'll get 0.
Code:
Number = !Number // Number is now 0 ("not 20")
Code:
Number = 20;
Number = !!Number; // Number is now 1 ("not 20" is 0, "not 0" is 1)
The "HOLDING_ENTS_ARRAY_SIZE" define is probably used to set the side of an array which holds the data of (all?) entities.
I'm not really familiar with "%", I'm afraid.
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