Depreciation
The accounting spread of an asset cost across its useful life. In a fleet fund it is also the mechanism that accumulates money toward replacement.
Depreciation allocates what an asset cost across the years it serves, rather than landing the whole amount in the year of purchase. In general accounting that is a reporting convention. In a fleet internal service fund it is closer to a savings plan, because the depreciation recovered through chargebacks is what builds the balance the next vehicle is bought from.
That gives the schedule real consequences. Depreciating over a longer life than the vehicle actually serves means the fund collects too slowly and comes up short at replacement. Depreciating faster than the asset is used overcharges departments in the near term. Neither error is visible in the year it is made.
The distinction worth holding on to is between book value and market value. What an asset is worth on the ledger and what it will fetch at auction are different numbers arrived at by different means, and only one of them turns into money at disposal.