The thin end of the sales funnel is closest to your bottom line. The clients
here have accepted your proposal in principle, and now you're talking out
the details. (Of course, few proposals get signed off as-is; that's why this
stage exists.)
Contract negotiations take time. You might have 4 submitted proposals
at 80% probability, but budgets get cut, people move jobs, and requirements
change. Watch the calendar; it's no use having a megabuck of projects in contract
negotiation at 80% probability if those projects sit in the funnel for six
months.
With a managed conversation over time, some proportion of those negotiations
will reach the end result: a sale. And that's your sales funnel.