Provisions the device with a CDimage build of Ubuntu Touch.
Provisions the device with legacy unflipped images. These images really shouldn't be used anymore, but it's a good time machine.
Provisions the device with an Ubuntu Image Based Upgrade image. If you still don't know what this is, refer to Stephane's blog post or the wiki describing it.
From the link:
Some of you may be aware that I along with Barry Warsaw and Ondrej Kubik have been working on image based upgrades for Ubuntu Touch. This is going to be the official method to update any Ubuntu Touch devices. When using this system, the system will effectively be read-only with updates being downloaded over the air from a central server and applied in a consistent way across all devices. Design details may be found at: http://wiki.ubuntu.com/ImageBasedUpgrades
After several months of careful design and implementation, we are now ready to get more testers. We are producing daily images for our 4 usual devices, Galaxy Nexus (maguro), Nexus 4 (mako), Nexus 7 (grouper) and Nexus 10 (manta). At this point, only those devices are supported. We’ll soon be working with the various ports to see how to get them running on the new system.
Provisions the device with a community supported build. Community based images as described by Daniel on his blog. In a nutshell, each porter provides a manifest of where their ports are located and the are provisioned to the target devices, this currently only works with recovery images that support extendedcommands. Support for openrecoverscript is one of the close targets.
ubuntu-system will be the official one. So this is better support, regular updates and easier setup.