What is the difference between a Standby and an understudy on Broadway?

A standby is an actor or actress who is ready to replace another actor or actress in a theatre performance if they become unable to perform due to illness , injury , or other emergency . While an understudy, is an actor or actress who learns a role in addition to his/her current role as a backup who can cover roles in case the main actor is unavailable . Understudies attend all rehearsals. Standbys are only required for performances. Usually, the understudy will perform smaller role(s) as well.