It's the first 3 classes of chanmodes, as defined in the CHANMODES token of raw 005. There are 4 classes that the CHANMODES token defines, separated by a comma (ASCII character 44 - the delimiter I'm using in that $gettok() call). The first two classes take a parameter when they're set (+
<mode>) and when they're unset (-
<mode>), the third class only takes a parameter when being set, and the fourth class never takes a parameter. If you take a look at the entire line that the $gettok() is being used on, it's checking that if the sign is a plus then the character is in the first 3 classes, and if the sign is negative then it's checking if the character is in the first 2 classes. In other words that line is checking whether that particular mode takes a parameter.
Note: Take a look
here for a more complete explanation of the CHANMODES token.
