To clarify:
RNG = Random Number Generator.
What people mean with this, is the following:
Imagine a boss dies. The RNG makes a variable and says:
If RNG=500, Then DropMount=Yes;
If Else StartScript:ScrewYou;
End;
No need for coding experience to understand that I hope. RNG picks a random number between 0 and 1001, and if the number happens to be 500, then the mount drops.
So yeah OP, Its Random