This means they are either produced for a limited time period, or an announced quantity is issued. Only Ski Country has consistently announced production quantities of its limited editions. Other companies have selectively released that information. Not all limited editions go up in value, some even go down. But knowing the production quantities for limited editions can be of great value.
Collectors usually start acquiring decanters quite by accident.
They might receive one as a gift, or purchase one that might be a particular interest. A specialized collection can take any form you wish. Some seek to own one of each piece a particular manufacture has created. Others specialize in sets or series within a manufacturer has created. You might want to cross manufactures lines and armed with a good reference book such as Mario's Bottle Guide decide to acquire all eagle decanters issued or perhaps all decanters with a transportation theme.