Do you mean the difference between "Status changed: Online" and "Signed In (Online)"?
If that is what you're talking about, then the only difference is that in the first case the contact was already signed in and had a status other than online or appear offline (e.g., busy), then changed the status to online. In the second case, the contact was either offline or appeared offline and now signed in or changed the status to online.
Although if that's not what you meant, then please specify...
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