Hook Points 
The system can execute hook point activities at several different points in time. These points in time are called hook points. Hook points allow you to precisely control when the system executes hook point activities.
The system has one set of hook points at the object level and another set at the site level.
Note
The system executes all code associated with a hook point in the same database transaction. For hook points within POD pushbutton activities, the transaction includes a single pushbutton activity, such as Start (PR500). If the hook point activity fails, the system rolls back, or cancels, the entire transaction. For example, in the figure in Site Level Hook Points, if you associate Check Configuration (CT520) with the POST_START hook point and the components have not been assembled, the system rolls back the Start as well. This is true for all hook points.