What is the difference between extending the project schedule and reopening a project?

Extending your project schedule will allow Vendors who have already submitted to change their submission (Can I un-submit my bid once it has been submitted and finalized?) but will also allow Vendors who have not yet submitted, to submit a proposal. See How do I change the Open/Close/Evaluation dates? to learn more about modifying your project schedule. 

Reopening a project will only allow Vendors who have already submitted to interact with your project. This is great for asking for more information from your vendors as needed, without opening the project to allow new vendor submissions. See Reopening the Project for more information.