Any kind of business or accounting system tends to be an endless long tail of "can the invoicing system send the invoice in Esperanto using only 16-bit Unicode characters on Tuesdays but only while it is raining and only for customers whose last names end with E and who signed up more than one year ago according to the Chinese calendar?" type features.

That's why attempts at "no-code" systems where biz/accounting people can design their own system are a perennial fad, but as these systems are developed they always just end up evolving into weird domain specific programming languages.

It's also why any sufficiently large company ends up implementing their own in-house accounting system and/or customizing some byzantine monstrosity like SAP which is really just another way of implementing your own.