Comment 8 for bug 1643732

If you are acting on behalf of other applications, you should use their applicationId; but in your case I believe that you really want your own applicationId. Let me try to explain why it's needed.

In the Online Accounts panel in the system settings, when the user clicks on one existing account, he'll see what applications can use this account, and he's got a chance to enable or disable them.

Now, you need to decide what the user should see in there, and how you want to present the storage-framework to the user: I think that the user should see something like "[icon] File synchronisation [enable/disable]", right?
This "File synchronisation" string, along with the icon next to it, comes from an .application file (it can be optionally overridden by a .service file, for those cases where you want to have different icons/text for different accounts); either storage-framework or some other app needs to provide it, and storage-framework needs to use this application-id to find out whether the synchronisation is enabled for that account.