Functional area derivation issues

We recently migrated to the new GL.

We are using functional area substitution and have found that in some cases the functional area is not derived correctly and have been searching for reasons. The rules have been in place for many years and always worked previously.

The instances where they do not work are for postings where the postings are to secondary cost elements rather than primary GL accounts.

We have located a configuration transaction FAREA_MODE. We have found that this seems to help in getting the rule to work. However, we do not know what this configuration was designed to do and what other impacts it could have. It can only be access using the tcode and it not in the IMG.

Has anyone used this configuration area (FAREA_MODE) with P&L accounts or does anyone know its intended purpose?

Thanks