A Full Accounting Unit (FAU) is the term used to refer to all of the numeric fields attached to the fund number under which cash is received and spent. Each field in the FAU represents a piece of information about the type of account. The FAU provides the who, what, and why for the award when it is established in the Financial System. More detailed information about an FAU can be found here.
Fom the UCLA Office of Research Administration training module website.