The main feature of the Commerce Invoice Module is dealing with Invoice Numbers when it's creates an invoice for Drupal Commerce Orders.
Some other modules like Commerce PDF Invoice or Commerce Invoice Receipt don't deal with invoice numbers. Commerce Invoices is doing a complete different job (see explanation).
The invoice handling in Commerce Invoice is realized with an own Invoice entity which connects to orders. Because of this there is a lot of flexibility to use Commerce Invoice in own views and rules, maybe for creating Invoice PDF's (see documentation).
Currently there is a simple possibility to choose how invoice numbers are generated. We are working on an improvement there with more flexibility and an interface for other modules to provide complex strategies on that (see discussion).
Note: the development version now integrates Commerce Invoice Email, which allows you to send an invoice by email thanks to the Commerce Email module. Note, however, that in order to use Commerce Invoice Email, you will need to use the 2.x version of Commerce Email.
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