First, here's an overall view of the back panel to give context with the rest of the iPB-10.
Now let's look at the I/O in detail, starting from the rear panel right and going left.
The power button and AC adapter input connectors probably don't need explanation. The USB 2.0 port provides an interface for recording directly into a DAW; it streams two channels to the computer simultaneously with receiving two channels from the DAW for monitoring what's been recorded. It runs at 44.1kHz and offers 24-bit resolution.
As to the rear panel footswitch jack, I haven't figured out what it's for yet, and I can't find a reference to it in the documentation. So either I'll figure it out, or someone from DigiTech will weigh in and let me know what's happening with it.
The headphone jack is an 1/8th-inch type--I'd prefer a 1/4" jack for studio-type headphones, although it seems these days most headphones come with adapters so they can be used with 1/8th-inch or 1/4-inch jacks. However, on the plus side there's a physical output level control. I think it's crucial for any piece of gear with a mostly virtual interface to have a way to get to the volume quickly so you don't have to go through menus or other hoops to set the levels.
Now let's move over to the left side of the panel.
There's actually quite a bit of flexibility going on here.
The XLR outs are ideal for feeding a mixer, and include non-defeatable cabinet emulation (what DigiTech calls "speaker compensation"). The 1/4" outs can feed amps or mixers; note the switch that optimizes these for amps for mixers.
Moving more to the left, there's a Stomp Box send/return loop that lets you put an external hardware device into the iPB-10 signal chain. This also has an associated footswitch on the top of the unit so you can bypass or enable it.
The Amp/Loop sent/return jacks are intended for amps with effects loop, and basically lets you use an amp's "front end" preamp/tone stack as another effect. The iPB-10 send jack patches to the amp's input, with the amp's effects loop send returning into the iPB-10 via the iPB-10 return jack. The iPB-10 output then goes into the amp's loop FX return. So, your signal flow is guitar > amp preamp > iPB-10 > amp power amp/cab. Like the Stomp loop, the Amp loop also has an associated bypass/enable footswitch on the top of the unit (you can see the Amp and Stomp footswitches to the left of the iPad bay in post #1).
Finally, there's an input jack (of course) for your guitar, and two ground lift switches for troubleshooting potential ground loop problems.
The bottom line is that the iPB-10 is clearly designed to fit into a variety of contexts, from direct feeds into mixers, recorders, or PA systems, to plugging into a guitar amp, or providing effects for an amp's loop.