Proper Definition of Terms Used in Guides

I find that the user guides do not do a good job of defining the terms that Practice Engine uses. Some terms are defined while others are not. One example is WIP Billed in the Billing Portfolio. What is this? I have a client where we have recorded WIP but have not cleared any WIP yet. Practice Engine still reports a "WIP Billed" amount. This is very confusing. No amount of searching in the guides and help center will tell me what this amount actually means. Iris needs to define every term regardless of how obvious they think the meaning is.