A project can have any number of estimates. Estimates can be for all or some of the total work of the project. Generally one estimate is created in response to an enquiry. Further estimates may be generated once the contract has been won in response to emergent work.
An estimate must belong to a project - indeed, it can only be created via its parent project. However estimates are completely self-contained. Their data is not linked to the project. To extract the data from an estimate, you import the estimate to create new tasks within the parent project.
Epic can store external and internal estimates.
An external estimate is simply a place-marker to one or more files created by another program such as a spreadsheet. You record these files as documents of the estimate.
An internal estimate is one that is created using Epic's estimating plug-in. There is an estimating tutorial explaining this.