This command defines a character set. The format for the charset command is
charset <name> = <character numbers>
For example, "charset first_pos = 1-720\3" defines a character set called "first_pos" that includes every third site from 1 to 720. The character set name cannot have any spaces in it. The slash (\) is a nifty way of telling the program to assign every third (or second, or fifth, or whatever) character to the character set. This option is best used not from the command line but rather as a line in the mrbayes block of a file. Note that you can use "." to stand in for the last character (e.g., charset 1-.\3).