Answering questions below.
Do I understand it correct when I think you can plug in an midi-over-usb device in the usb port, and it will generate CV?
Yes, this is the point of the module.
You can either (just NOT at the same time!):
a) plug into your PC/DAW/IPAD. The E620 is a Device, the PC side is the Host.
b) Plug a MIDI USB keyboard into the E620. The E620 is the Host and powers the Device.
Note: a Monome is NOT a Class Compliant device. You have to buy the White Whale modules for it.
Are you able to run audio from an ipad for example through one of the cv outputs,
No, that is a different USB Class (the Audio Class). And that class is pretty restrictive in terms of sample rates/bit lengths. Which is why MOST USB audio interfaces use their own custom drivers.
So no concept of "MIDI thru" where USB is concerned? (I'm no expert)
This is what a USB hub is for in Host mode. The PC (Host) sends 1 USB MIDI stream out it's USB port to the hub. The hub (usually 4 ports) then sends it out the 4 ports (like a mult). So there is no need for 'Thru'
I wonder that, too. Would like to know the difference between host and device mode.
This is USB terminology not a MIDI term. A Host can provide power to the Device. The Host is usually a PC/DAW. The Device is like a USB mouse, thumb drive or in our SPECIFIC case, a MIDI thing (like my M-Audio keyboard).
Either Host or Device can send and receive USB data.
I thought that, in order to send MIDI over USB, we needed some kind of software
It depends if the USB Host/Device is 'Class Compliant' or not. Again, an official USB term.
The USB spec has pre-defined Classes. The intent is that these devices need no additional driver. All the messages are pre-defined, you cannot add to them. The E620 is such a device, the 'drivers' are executing inside the PIC microcontroller. This means that we cannot add to the USB commands for the MIDI class.
Now, if your Host application has it's own "MIDI driver" for say sending MIDI, what does the E620 do in this case?
The messages added to the MIDI portion are ignored. However, the MIDI class is pretty much the entire MIDI spec so I doubt any functions are 'lost'. The added drivers are usually for the Audio side of things.
Official spec is here:
http://www.usb.org/developers/docs/devclass_docs/