From the Lotus Domino Administrator 6 Help database:
There are two kinds of ECLs: the Administration ECL, which resides in the Domino Directory (NAMES.NSF), and the workstation ECL, which is stored in the user's Personal Address Book (NAMES.NSF). The Administration ECL is the template for all workstation ECLs. The workstation ECL is created when the Notes client is first installed. The Setup program copies the administration ECL from the Domino Directory to the Notes client to create the workstation ECL.