Does the license belong to the registered user (meaning the registered user is the only person who can use the software), or a machine (meaning the software can be shared with other people who use the machine)?
The software belongs to a single registered user. The end-user may have multiple installations, but only on the machines that he/she has exclusive access and control on. The software can't be shared.
On a separate note: we also offer a classroom site license for 25+ licenses (or seats), in this case, the software is installed on all lab machines, and students of the class can use the software on any machine. For this type of license, the cost of this type of license is determined by the number of seats: