When you install a card in slot 3, it can be configured into either NORMal or INSert modes.
In NORMal mode, you can send any signal (1-32, aux sends, buses, L,R) to any of the eight outputs, exactly like the To-Slot capability of the Slot 1 and 2 cards. Inputs from the Slot3 card either go nowhere, or to channels 9-16, depending on how you set them in the other DIO page that shows the routing schematic. There are no other choices for routing Slot3 inputs: either the entire block of eight inputs replaces analog inputs 9-16, or they go nowhere.
In summary, you route any signal to any Slot3 output. You separately route all Slot3 inputs, as a block, to either analog input 9-16 or nowhere.
In INSert mode, Slot3 operates as eight insert send/return pairs for aux sends, buses, L, or R. (But, sadly, NOT for 1-32). Each of the eight connector pairs can be configured as an insert for a bus, or L, or R. When you configure an IO pair this way, the DA7 automatically sets the input channel to come back to the bus, L or R it's sending---thus it's a true insert.
Auxes in INSert mode
In INSert mode, you can also assign an aux SEND to a Slot3 output, and the corresponding input will automatically be routed to the corresponding aux RETURN Thus that IO pair does not become a true insert, but rather a replacement aux send/return jack pair. For example: you select Aux Send 2 for output #1. Input #1 is automatically routed to Aux Return 2. Now you have an analog aux send/return for Aux 2.
(I haven't tested what happens if you try to bring a signal into a "real" aux return jack at the same time, but I'll bet the card simply overrides that jack signal.)
(The DA7 prevents you from setting up multiple IOs as the same insert function, which would be unusual. (e.g. IO #1 and 2 BOTH as inserts for Bus 1). It wouldn't make much sense to do this anyway.)
In summary, you configure an output and its corresponding input to act as an insert for bus 1-8, or L or R main mix. Or, you can configure an output/input pair to act as an aux send/return jack pair.