Multi-round bidding becomes useful when there's a time gap between the original bids vs when the job actually starts. Meaning that the prices or the availability of subcontractors might have changed.
Creating a new round
Open up the original bidding table and click on "Create new round".
This will create a new RFQ based on the original one. You can change subcontractors, description, line-items etc. before sending it out.
Once you do send it out, the old and new RFQs will be linked together. You can access the old round by clicking on "Previous round".
Alternatively, you can just create a copy of the RFQ and modify the copy before sending it out. In that case the RFQs won't be linked together.