"Assigned": A new lead is captured and assigned to you.
At this point, you can start acting on the lead by reaching out to the potential prospect. Set the status to "Working."
Keep working on the lead until you are ready to send a proposal to the prospect. At this point set the status of the lead to "Proposal Sent".
Pending the prospect's decision, update the lead status. If the prospect accepts the proposal, set the status to "Completed". Alternatively, you can drop the lead as a "Closed-Lost" opportunity or "Archive" the lead if it needs to be put on hold.
The proposal has been accepted by the client! Now your management will review and provide final approval about the status of the lead.