When used with 2 arguments, e.g. `a ≢ b`, it indeed is called "not match", i.e. are `a` and `b` the same. When used with 1 argument, `≢ a` it will give you the number of elements.
Yeah, the fact that the same symbol is overloaded with two meanings for the one-arg and two-arg (infix) cases adds some overhead to reading APL code. The precedence rules are very simple and uniform in isolation, but personally I find that applying them to (mentally) parse a nontrivial line of code is one of the hardest parts of getting started with APL.
