For the current synchronous grammars based SMT, to some extent, the generalization ability of the grammar rules (the usability of the rules for the new sentences) can be considered as a kind of the generative power of the grammar and the disam-biguition ability to the rule candidates can be considered as an embodiment of expressive power.
the generative power refers to the ability to generate the strings of the language, and the expressive power to the ability to describe the same language with fewer or no extra ambiguities.