Icon sets are a great way to segregate data with an identifier based on thresholds. For example, if we can say that if at any time revenue is above a certain number, we're doing fine. If it's between two numbers, we're worried, and if it's below a certain number, we're in trouble. Icon sets let you represent data like this graphically. A firm might say that monthly revenue over 1,000,000 is fine, revenue between 750,000 and 1,000,000 is okay, and revenue under 750,000 requires management to intervene to find out what's going on.
The first tier can be represented by a green circle, the middle tier by a yellow triangle, and the last tier by a red diamond. This gives anyone even glancing at the dashboard a good idea of what needs their attention.