The client supplied the domain knowledge...in more than a few cases they were doing things manually, and I automated them. For example the vendor verification involved manually looking up licenses, or doing data entry on ACORD25 insurance forms. I automated it, so that given a license number, trade, and state, a crawler would fetch the data on a schedule. I also wrote a parser that could read insurance data from an ACORD25 form and put it in a database. The client was tracking all of this in spreadsheets, and I just copied the logic from there.