BRE in R2 environment
Rules are User-defined custom criteria which trigger sending Alert or Notification, either through On-Screen Display and/or an E-mail. These trigger help the user be informed about changes to data.
For Example:
In R2 Core, this could be about changes pertaining to an Order, Non-availability of an Item, Invoicing of an Order, etc.
In R2 Labor, this could relate to notifying a set of users about a labor position being modified.
In R2 CRM, this ca help setup rule about informing some users about Creation of a new Company or Opportunity, or changes in status of them.