Opportunities
Opportunities are entities that allow you to track the status of a lead from start to finish. They are required as part of the Account - Opportunity - Estimation process.
Opportunities can be created right after creating an account, and used to track everything from the first phone call and initial meeting to revising and eventually winning the estimate. Alternatively, they can be created just before creating an Estimate.
Regardless of the process, the creation of an Opportunity is required. All Estimates will be tied to an opportunity, and the opportunity is meant to keep track of the current stage of that lead.