The Size Percentages fields are used to specify the estimated amount of new, modified, or unmodified code in the project. You will only see these fields when the size method is set to Direct or Calculator (Size bin estimates are set automatically to 100% New).
The percentages you provide here will be applied to the overall size estimate and the percentage values must sum to 100%. For example, if your total size estimate (whether entered Directly or via the Size Calculator) is 100,000 Total Implementation units and you specify a breakout of 50% New, 25% Modified, and 25% Unmodified Code, the calculated size used to select a PI rank and to position the estimate against your chosen trend group would be:
50,000 New + 25,000 Modified = 75,000 Effective (New + Modified) Implementation Units.
Unmodified code will not affect either the estimated PI or project risk ratings but can be recorded for analysis and documentation purposes.