When shown as zu2 or zu3, etc., it is a directive to indicate the number of musicians to perform the indicated passage of music. The musicians would perform the indicated passage in unison with two (2), three (3), or the number of musicians indicated. This is most often used with stringed instruments (i.e. violins, violas, cellos, basses), but can be used for any instrument in the ensemble where there are multiple musicians performing one part.