What is silver recovery in film processing?

The silver halides in the emulsion which were not reduced during development, are
dissolved in the fixer. Silver can be recovered from the fixer in order to keep the silver
content of the fixer solution as low as possible so that the fixer lasts two to four times
longer, and sell the silver.
Silver recovery can, for example, be done by electrolysis. In addition to electrolysis
equipment, there are other silver recovery systems commercially available.
It is worthwhile considering subcontracting this work to a specialised firm in view of
secondary aspects such as organisation, logistics, storage and environmental
requirements.

What are relative exposure factors?