Security equivalence means having the same trustee assignments as another object. When you make one object security equivalent to another object, the rights of the second object are added to the rights of the first object.
For instance, suppose you make User object Joe security equivalent to the Admin object. After you create the security equivalence, Joe has the same rights to the NDS tree as Admin.
NOTE: Security equivalence is only effective for one step. For instance, if you make a third user security equivalent to Joe in the example above, that user will not receive Admin rights.
Security equivalence is recorded in NDS as values in the User object's Security Equal To property.
When you add a User object as an occupant to an Organizational Role object, that User automatically becomes security equivalent to the Organizational Role object. The same is true when a User becomes a member of a Group object.