How to describe free capacity

How to describe free capacity?

I often see ads or message with the message “We have free capacity” or “We have free seats”.

Is there a way to describe the capacity more sharply defined?

I have seen things like “In the next 10 Days we have 20 seats with 360 minutes outbound capacity.” In the market research field some firms describe there capacity with the quantity of interviews they can conduct in the next days.

In one Question:

Is there a standard, a common way, a norm or something else to describe capacity of callcenters? If yes, why isn’t it used when posting ads.

I am interested in your thoughts!