Hi Rebecca,
The engagemnet stream does not keep track of what spot in the stream a lead is in. When a cast goes out, the leads will be sent the peice of active content, closest to the top, that they have not received yet.
So, if the next peice of content is deactivated for a lead, they will skip it and be sent the following peice of content in the same cast. If you reactivate the email later, the lead will receive that email in its next cast.
If a lead is not exhausted, they will always receive an email with every cast.
Does that answer your question?
Thanks!
Will