Format
A := B
This is the “assign into” operator. The value of B is evaluated, then A. If A is a datum that has an operator:= proc overloading this operator, then that proc will be called with A as its src and B as its only argument. The return value of the proc (which defaults to its src, the old value of A) is assigned into the var that holds A.
If A was not a datum, then B is assigned into the var as if this were an ordinary A = B assignment.
A common use of this operator might be to copy another datum. This is basically just “syntactic sugar” to make certain datums easier to work with, and is intended mainly for situations where you’ve overloaded the operator.