WordNet sense 1 (added to a regular schedule):

extra

WordNet sense 2 (not scheduled or not on a regular schedule):

unscheduled

WordNet sense 3 (developed or designed for a special activity or function):

specialized, specialised

WordNet sense 4 (first and most important):

particular

WordNet sense 5 (of first rank or importance or value; direct and immediate rather than secondhand):

primary

WordNet sense 6 (beyond what is ordinary or usual; highly unusual or exceptional or remarkable):

extraordinary

WordNet sense 7 (unique or specific to a person or thing or category):

peculiar

WordNet sense 8 (having a specific function or scope):

limited

WordNet sense 9 ((sometimes followed by `to') applying to or characterized by or distinguishing something particular or special or unique):

specific

WordNet sense 10 (surpassing what is common or usual or expected):

exceptional, especial

WordNet sense 11 (not common or ordinarily encountered; unusually great in amount or remarkable in character or kind):

uncommon