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From the 2018 UPC Guide to Important Code Changes

What Changed.
610.9 Size of Branches. Where Table 610.4 is used, the minimum size of each branch shall be determined by the number of total fixture units to be served by that branch, and then following the steps in the total developed length of the system, and the meter and street service size in accordance with Section 610.8. No branch piping shall exceed the total demand in fixture units for the system computed from Table 610.3. is required to be larger in size than that required by Table 610.4 for the building supply pipe.

Why It Changed.
The goal was to simplify water pipe sizing. The UPC Study Guide notes the previous method "inflated the fixture unit value" and the Illustrated Training Manual notes the previous method "results in fixture unit totals exceeding the original demand for the building." Both publications note that there is a potential for confusion using the previous method of assigning fixture units to a branch when the system has both hot and cold water outlets.

A code change was submitted stating that most of the confusion comes when you can, and do, have more fixtures units on a branch than the total system fixture units at the meter or for the building supply serving such branches. However, that happened regularly with the previous method shown in 610.9. With the new language you stop adding fixture units when you reach the total system fixture units you've previously computed for the water meter and building supply, and maintain that total to the meter. Now there are no "inflated fixture unit values," while the pipe sizes remain the same as they would have been using the previous method.

What It Means to Me.
Engineers and designers using Table 610.4 can more easily select pipe sizes based on accurate fixture units counts.

Plans examiners and inspectors need to understand and implement this new step-by-step process of assigning fixture unit values to sections of piping while not exceeding the total demand at the meter.

Plumbers, installers or technicians also need to be aware of and trained in this new way of counting fixture unit demand in a distribution systems and quit adding FU when you reach the total demand determined for the meter or source of supply.