When you choose to add the expenses to the invoice you can select and add only the expenses you want to associate with that invoice, but only defined by details, they could be of a number of mixed projects for that client, so impossible to say if you are adding the right ones. There is a project link too, but for me it only works if ALL the expenses within a the project are unbilled. If some expenses are left over from a project, they are listed together. Some more detail would be so useful to have so the selection is possible, it is as nice function as useless to me as it is now.