See the proc directive in the MASM reference manual for details. The scope of a name limits its visibility within a program. That is, a program has access to a variable name only within that name's scope. Outside the scope, the program cannot access that name. As you've seen, MASM provides some minimal scoping features for statement labels.
Address binding is the process of associating a memory address with a variable name.
Variable lifetime is that portion of a program's execution during which a memory location is bound to a variable. Unit Activation, Address Binding, and Variable Lifetime Unit activation is the process of calling a procedure or function. The combination of an activation record and some executing code is considered an instance of a routine. When unit activation occurs a routine binds machine addresses to its local variables. The lifetime of those variables is from that point until the routine destroys the activation record eliminating the local variable storage.