Yes I understand how software development works. However, when you are usually working with a budget you can afford to hire developers that know how to code stuff. I'm not a developer. So I have to make sure that the way I design it, I can actually code. I know how to design it properly so that it can be built, but if I CAN develop it that way it a totally different story. So I did my design, and I'm making sure I can do it, then I will do my formal documentation. Instead of doing all of the formal documentation before I start development. Just leaving myself some flexibility that you otherwise wouldn't have is this were a real development project.

At this stage it seems to me that you don't have any "products" you actually have "contracts" which consist of "jobs" which consist of "work packages" or "tasks" .................you can break it down further if required.
My table stucture is already built. These are invoices, and for this project, my product is services. So you can bill it out as a part. For instance, for a specific job you may need 10 linear feet of concrete. On the invoice that will be 10 units of 1 linear sq.ft. So yeah, I'm using "parts" interchangably with "tasks" or "services." Although he could also really sell a part in the future and it would be tracked the same. As it is now he doesn't buy anything, he just shows up and does labor. He goes to his supplier, orders the stuff he needs to do his work, and all of the gets billed directly to the pool company. He doesn't have to manage any of that.

So, another question: Do you bill only on completion, or are there progress payments?
This app will only manage invoices. No payment information will be handled. He has his payment stuff all worked out with his accountant and I don't want to get involved with what they are already doing. Maybe in the future, but not for what I'm doing with this class. I have a time constraint that I'm working under, so making it a complete business management system isn't really an option.

1. This is a "subcontractor"................what does that mean? do they work for a main contractor, or directly for the customer? (sorry I don't know US constructiopn terminology)
OK, so a home owner wants a pool. They go to the pool company and order what they want. That pool company then pays subscontractors to go and actually do the work. One contractor may dig the hole, another one puts the steal rebar for the shell, another one does the concrete, etc.. This particular company I'm working with does concrete, plumbing, tile, and stone work. Although none of it really matters to the application as it is only concerned with units or parts.