The "Combine Email Documents" function does NOT combine several reports from report selection!
User Story: In Switzerland we typically have two reports in Sales Invoice Report Selection. The first Report is the Invoice (such as Report 1306) and the second one is the payment slip (R 11510).
What we would like to have when we send the invoice by e-mail is to have ONE email containing two attachments: The Invoice and the payment slip. Or even better: One PDF as combination of Invoice+Payment slip.
The "Combine Email Documents" doesn't do that.
What it does is something different:
Let's assume a customer has three invoices we want to send in ONE email. We would select (mark) the three invoices from the list of posted sales invoices and start the action "send" with the Combine option enabled.
BC would then start the invoice report once with a filter on "Marked=TRUE" thus starting the report for these 3 invoices in one pdf.