Q:

What is a rhetorical concession? 

For context:

The expression is used to bring up an unsatisfactory aspect of the seemingly agreed statement, thus it is a rhetorical concession. 

It's explaining a construct in Chinese Mandarin. 

A:

Here's an explicit definition of a rhetorical concession. 
In oratory, rhetorical elements are only there for the sake of effect - expressed in terms intended to persuade or impress.  So in the context of formal agreements, a rhetorical concession is one which actually concedes nothing of substance, yet gives the outward appearance of doing so. 
In essence, it's like saying "Yes, I accept what you say/want, but..." - then proceeding to argue against the point you've apparently conceded.  For example, this statement made on behalf of employers using badly-paid "sweat-shop" labour:
"Employers recognize that wages are essential to meeting employees' basic needs"
...as a precursor to an "agreement" which concedes nothing in respect of working conditions/pay.