Question: Is there a difference between procedures and options in commands

This is for my understanding (and the proper use of Maple terms)

?simplify refers to them (sqrt in this case) as procedures

simplify[sqrt](16^(3/2));
                               64

`simplify/sqrt`(16^(3/2));
                               64

simplify(16^(3/2), sqrt);
                               64

?combine calls them (this time exp and trig) names of options

combine(exp(sin(a)*cos(b))*exp(cos(a)*sin(b)),[trig,exp]);#why the list?
                        exp(sin(a + b))

combine(exp(sin(a)*cos(b))*exp(cos(a)*sin(b)),trig,exp);#no list
                        exp(sin(a + b))

combine[trig](exp(sin(a)*cos(b))*exp(cos(a)*sin(b)));#no exp required?!?
                        exp(sin(a + b))

(`combine/exp`@`combine/trig`)(exp(sin(a)*cos(b))*exp(cos(a)*sin(b)));
                        exp(sin(a + b))

If the terms command options and command procedures can be used interchangeably, how does evalf[4](...) fit into this scheme? 

Is there a special Maple term for the construct "proc[n]" where proc is a procedure/command name and n is not a name but of type numeric?