MDQ = Multi Dimensional Quoting. This is a way of selling subscription products, in a way you are able to define the price, quantity, uplift, discount different per each segment of time.
Without MDQ, for a 3 year term, the quantity, discount is the same for the whole subscription duration. When using MDQ with a yearly segments, you can define different parameters per each year.
Watch the short (2:06 min) video from Salesforce about the MDQ functionality.
