Your Nylas ID is used to manage your Nylas Pro subscription and support requests, and any email accounts you sync in N1 will be associated with that Nylas ID. You can check see many accounts are associated with your Nylas ID at billing.nylas.com/dashboard.
If you have previously installed N1 and synced multiple email accounts, your synced accounts will still be available in that N1 install once you upgrade N1 and create a Nylas ID—you won’t need to re-link them again.
However, if you run a fresh install of N1 on a new computer, any email addresses you have set up in the past will not be automatically added to the new instance of N1. You will need to set up each email account separately.