The dictionary is what lets a message be chosen by device and parameter name instead of by CC number. It is held on the unit, and Studio updates it.
It currently holds 112 manufacturers, 482 devices and 17,308 parameters.
Studio also takes entries of your own, for a device that is not in the list yet or for a parameter that a manufacturer documented after the last update. Entries you add stay yours and are not overwritten by an update.
A device worth adding is usually worth reporting in Works with as well, so it reaches everyone’s dictionary rather than one unit.