In the scenario you suggested it would only be possible for you to match
the Star Trek, Group: Jadzia & Janeway, CH: DS9. The use of canon helpers
would prevent you from being matched to the CH: Voy one. The use of CHs is
to let both people that want to have very specific parts of a fandom and
very broad parts of a fandom both be accommodated. So if you picked the
relevant characters and then CH: DS9 you'd only be matched with people who
were also into DS9. Now, say you requested CH: DS9 specifically, and
someone else who knew all of Trek offered all three of CH: TNG, CH: DS9,
CH: VOY they could still be matched to write for you but they'd know that
you only wanted DS9. While somebody that was only into TNG couldn't be
matched to you at all.
Re: Star Trek
In the scenario you suggested it would only be possible for you to match the Star Trek, Group: Jadzia & Janeway, CH: DS9. The use of canon helpers would prevent you from being matched to the CH: Voy one. The use of CHs is to let both people that want to have very specific parts of a fandom and very broad parts of a fandom both be accommodated. So if you picked the relevant characters and then CH: DS9 you'd only be matched with people who were also into DS9. Now, say you requested CH: DS9 specifically, and someone else who knew all of Trek offered all three of CH: TNG, CH: DS9, CH: VOY they could still be matched to write for you but they'd know that you only wanted DS9. While somebody that was only into TNG couldn't be matched to you at all.