 Historically, this term is commonly used in IBM mainframe vernacular.If the user site specific code is substituted for the software vendor provided default exit it must interface to the software package using the defined parameters as documented for the default exit.User exits are important because while they enable site specific customization they isolate such installation specific customization to defined and supported points enabling the site to upgrade to follow-on releases of the software package without adverse impact to preexisting customized functionality.