At this point, only some rule of type (26) can be applied, resulting in the reduction of the stack, immediately followed by some rule of type (25), which is a composition rule of degree m. If the derivation were to use more than one occurrence of (26), then it would derive a category whose stack contains fewer than m elements.
rule (25) would no longer be applicable, because of the restriction on the composition degree, and the whole derivation would come to a dead end.