What I found missing from the case study is billing or POS (Point of Sale) which would draw data from the different tables to produce a final invoice. All the information is recorded. Do we need to worry about an invoice/cash receipt table for instance? I am thinking from a process point of view that this data would be mined for billing purposes.

I think it’s great that you are considering the billing and invoicing aspect. It makes sense to include an invoice or billing table to link all the relevant data together (e.g., client, vehicle, tariffs, number of days hired, insurance details),which is often a critical business need for tracking payments and managing accounts.You can add it to your design and explain this assumption in the optional assumption document that the project allows.

If it is not required then I am not going to put it in. I guess I was just thinking about it from a business perspective and I thought, is this the missing link? That leads to a HD? The obvious one that we need to put it. Point of Sale I guess is the most crucial part of the whole model, but you have confirmed that it is optional.

Yes, that is a very good point! Though you do not have to include invoice right now, in the second part, I'll show that business requirements can change and suddenly the client realises that they have missed some requirements. Invoice is coming in part 2. Also, note it is a good thing to ponder over missing entities, but also keep in mind that you need to work within the scope of the case study :D