How billable Items are meant to be used in a work order

AppFolio user question:

Can anyone give me a clear example of how Billable Items is meant to be used in a work order? It's not making any sense to me at all.

APM Help answer:

Billable items can be things like labor and supplies that will add up to be the grand total for the amount of the work order bill. Ex, one billable item could be 2 hours of electrical labor and another billable item could be whatever supplies were purchased for repairs). Either the owner or tenant will pay for the billable items on the work order depending on how they generate the bill.