I believe this is where you would Link the account. When you start Warframe on PC you would link you Xbox as the main account so that your Xbox account overwrites the PC account and you can play the Xbox account on your PC. There are other replies that explain it better, but I figured with the update actually out everything will be fully explained next week.
edit: I forgot to mention that linking accounts doesn't apply to the NOV 24 dateline, that was for merging which is combining two accounts into one