Revit performs a general conversion from fixture units to flow, using the values found in the 2006 International Plumbing Code (IPC), Table E103.3 (3).
The flow conversion method selected in the Instance Properties dialog for the selected system determines the section of the IPC table used for the conversion. The resulting flow is used to calculate pipe sizing.
The value calculated for flow can be helpful in sizing the branch piping for subsystems. However, when specifying sizes for the main piping, you should consider a variety of other factors, such as: