You can create Custom Sources which are variables that you define for easier reference and repeated use of a particular Custom String.

For example, you may have created a Custom String such as max(dbmarket, dbregionmarketavg) - instead of typing this out every time, you can create a Custom Source for this string and name it 'myprice' or something similar. Now when you enter 'myprice' in a Custom String it will refer to max(dbmarket, dbregionmarketavg).