This command defines a taxon set. The format for the taxset command is
taxset <name> = <taxon names or numbers>
For example, "taxset apes = Homo Pan Gorilla Orang gibbon" defines a taxon set called "apes" that includes five taxa (namely, apes). You can assign up to 30 taxon sets. This option is best used not from the command line but rather as a line in the mrbayes block of a file.