What does the term "top-most" mean in the context of formal grammars?
In the context of context-free languages, &quot;top-most&quot; refers to the root of a parse tree. Your rules guarantee that a sum of the form $1+1+1$ is parsed as $1+(1+1)$ rather than the opposite, since the nonterminal Ntp (&quot;not top-most&quot;) doesn't allow + to be the top-most operator (you should imagine a parse tree in which + labels the root).
Consider the following two parse trees: (generated using Syntax Tree Generator)
 
On the left, the left child of the root has + as the top-most operator.
On the right, the right child of the root has + as the top-most operator.
Your grammar only allows the second parse tree, since the left child of + must not have + as the top-most operator.