What is the price definition tag


I have some promos setup eg 2 cocktails for £8, but i only want that promo to work when the prices are set to a certain price definition.

When i use another price definition where prices may be more or less i will use another rule as then 2 cocktails may be cheaper or more than £8

How can i set my promo rule to only fire when only a certain price definition is set? I saw the {PRICE:x} tag and thought it might be that but it doesnt do anything when replace x with the name of a price definition