I am looking for the best way to keep track of the business rules for both developers and everybody else (support staff / management) in a startup enviroment. The challenge is that our business model requires quite a lot of different business rules, which are created pretty much on the fly and evolving organically after that. After running this project for 3+ years, we have so many of such rules that often the only way to be sure about what the application is supposed to do in a certain situation is to go find the module responsible for that process and analyze its code and comments. That is all fine as long as you have one single developer who created the entire application from the scratch, but every new developer needs to go over pretty much entire codebase in order to understand how the application works. Even bigger problem is that non technical employees don't even have that option and therefore are forced to ask me pretty much every day how some certain case would be handled by the application.
Quick example - we only start charging for our customer campaigns once they have been active for at least 72 hours, but at the same time we stop creating invoices for campaigns that belong to insolvent accounts and close such accounts within a month of the first failed charge. That does not apply to accounts that are set to "non-chargeable" which most commonly belongs to us since we are using the service ourselves. The invoices are created on the 1st of each month and include charges from the previous month + any current balance that the account might have. However, some customers are charged only 4 days after their invoice has been generated due to issues with their billing department. In addition to that, invoices are also created when customer deactivates his campaign, but that can only be done once the campaign is not longer under mandatory 6 month contract, unless account manager approves early deactivation.
I know, that's quite a lot of rules that need to be taken into account when answering a question "when do we bill our customers", but actually I could still append an asterisk at the end of each sentence in order to disclose some rare exceptions. Of course, it would be easiest just to keep the business rules to the minimum, but we need to adapt to changing marketplace - i.e. less than a year ago we had no contracts whatsoever.
One idea that I had so far was a simplistic wiki with categories corresponding to areas such as "Account activation", "Invoicing", "Collection procedures" and so on. Another idea would be to have giant interactive flowchart showing the entire customer "life cycle" from prospecting to account deactivation.
What are your experiences / suggestions?