QR codes for events

I want to generate (or use existing QR codes) for each person who registers for sessions within an event (each person will have ONE QR code, but can sign up for multiple sessions). We need those QR codes to generated before we send a confirmation email and then create a pdf for print out. The QR codes will be used for check-in at each session that someone attends. My client is using a company for the QR code scanning and I believe they want a certain number passed through when scanning the QR code that they then will put into a spreadsheet and deliver to us post event so we can upload the attendees. 

Can someone walk me through how this will work? I'm pretty confused on the process. 

Thank you!!