a "specialist" is someone who always, always plays the same type of character.
for example: in a modern day scenario, he plays a ninja. in a sci fi game, he's playing the stealthy intrusion cyborg. in a fantasy game, he's playing the assassin-rogue with mystical abilities. in a superhero game, he's playing a super-ninja. in a Western game, he asks if he can play a ninja who's migrated over from Japan.
or the guy who always plays honorable, law-abiding, dispensers of justice, regardless of campaign or scenario.
or the girl who always plays cat-beings.
that's what he means by a "specialist."