If the flights are eligible, to ensure you earn frequent flyer credit from the airline, give your frequent flyer account number when you check in for your flights. (singlestravelintl.com)
You will need to link the appropriate frequent flyer account number the first time you transfer to an airline. (upgradedpoints.com)
Your MileagePlus number is the frequent flyer account number assigned to you for signing up to the reward program. (upgradedpoints.com)