Clients
A client is a Mac or handheld device that is set up (or registered) to sync with your database. To view a list of your clients and each client's sync status, just choose Preferences from the OmniFocus menu, then click the Sync button, followed by the Show Clients button.
To synchronize changes between clients, OmniFocus has to keep a history of those changes which might not have been seen yet by every client. The more history OmniFocus has to maintain, the slower syncing becomes on every machine—so it's a good idea to try to synchronize every client on a regular basis.
If you're not going to use a particular client for a while and aren't worried about preserving sync history for it, you can unregister it to make syncing faster for all the other clients. To unregister a client:
Select the client from the list in the clients sheet.
Click the Unregister button.
If you unregister a client, OmniFocus has no way to know whether outstanding changes on that out-of-sync client might be newer or older than the changes made on other machines (since they no longer have a record of what's newer and older). The upshot of this is that the next time you sync that client, you'll be asked to make a choice between the data on the server or the data on that client.