Professional and Enterprise licenses vary in both license usage and deployment options.
License Usage
MonoTouch Professional and Mono for Android Professional are both licensed for use by the original user of the tools. This means that the first user to activate the tools is the only developer licensed to use the tools.
MonoTouch Enterprise and Mono for Android Enterprise are both seat licensed. Seat licensing allows the owner of the license to change the named user of the tools as developers on a project change.
Deployment Options
Both Professional and Enterprise licenses allow for ad hoc deployments directly to fewer than one hundred devices, and deployment to an unlimited number of devices through a public app store (such as Apple's App Store, Android Market, Amazon Appstore, or Barnes and Noble NOOK Store).
Additionally, our Enterprise licenses support large deployments directly to end-user devices (i.e., deployments that do not go through a public app store).
On MonoTouch, this refers explicitly to Apple's In-House deployment option available to members of their Enterprise Developer program: