Expense reimbursement

For me, you will not create a supplier invoice for the expenses. The expenses is enough and the invoice will not be addressed to the company otherwise it is not an employee expense but just a normal supplier invoice.

This is the main target of employee expense.

I would say that we should not have to decide on the module but the users will decide what can be used as expense or not.

I do not think we should extend the invoice types. Employee expenses will behave quite differently than an invoice, especially on the workflow.
So I think we need a new model.

That’s the tricky part. Often for small amounts (or foreign tickets) the taxes are not deducted because it is too much work for a small amount. Also there are cases where taxes are deducted only partially (usually fuel).
The difficulty is that product has for now only one definition a taxes for purchase but these taxes are not necessary the right one for employee expense. I think the best is that to use a product for employee expense, it should be flagged for (like salable for sale and purchasable for purchase). This way, users may create one or two product for the same thing depending if it needs different taxes/accounts per usage.