This process is described in Lotus Notes Help for sure, however I personally didn't know about it for quite a while, until go to a point on how to synchronize my local Lotus Notes contacts at work and at home. So I guess there are some other Lotus Notes user, which don't know about it yet, and hence it won't hurt to explain it here.
So, here is description how to synchronize your local contacts (usually names.nsf) and iNotes contacts stored in your mail file.
If you open your mail database in Lotus Notes client, you can see there is a switcher in left top corner to "Switch to Contacts", this is actually a way to see your server contacts:

They can also be used for addressing, when you access your server mail file via the Web (aka iNotes).
To enable the contacts sync between your local and server contacts, you got to go through 2 simple steps:
- Open your mail file Preferences, switch to "Access & Delegation" tab, and click the link "Enable 'Synchronize Contacts' on the Replicator". In the dialog window shown enable the corresponding checkbox and save the settings.

- Then you have to go to the Replicator and enable corresponding record there.

So, now you can replicate manually if needed, and if you want for this process to occur automatically, just enable scheduled replication in your location.

And again just to wrap up, why this might be useful:
- If you want to sync your local personal contacts with server mail file, for example to have them available for iNotes or make accessible to other users
- If you want to sync multiple Lotus Notes local address books between each other. Just perform these steps on all of them, and you have the same contacts everywhere synced through your server mail file.
