An operator, say $+$, is associative if $a + (b+c)$ means the same as $(a+b) + c$.
Rewriting this rule in RPN, we'd get that $+a+bc$ means the same as $++abc$.
In this context, may I assert that an associativity, in a formal grammar sense, means that $+a+$ is equivalent to $++a$, even if $a$ is a nonterminal?