Variable names uniquely identify variables. Multiple icons on the canvas with the same name at the same level all refer to the same variable. Variable names can have local scope, in which case it is specific to the group in which it is contained, otherwise its scope is determined by the innermost containing group that has a local variable of that name. If no such variable exists, its scope is global.
A variable which is local to a group can be used as a formula in
several locations in a file. For example, if you use a, b and c as
constants in a polynomial 
and make them local to a
group, then the group can be used in several parts of a document, with
the argument x and the values of a,b and c being different in every
instance.