Good you found the mailing list! Transaction classes and types have been introduced some time ago, but still lack good documentation (as far as I know). They are indeed used to distinguish between something like sub-accounts for a member, so that e.g. order payments and member fees are kept separately (and payed separately).
Please note that these features are not yet released, so you’d have to run the development version of Foodsoft (from master).
This was all written by @paroga, would he be able to explain this a bit more?