We recently updated the way our Importer handles Remote IDs. Before, creating a Remote ID for an existing profile was only possible when jumping through a bunch of hoops (which usually led to a headache). If you had a profile in your CSV that matched an existing profile by name and email, you could not create a Remote ID for that person.
All that has changed! Now, if you have an existing profile, and your CSV has a matching profile (by name and email) with a Remote ID, the Remote ID will be added to that existing profile.
The Remote ID is an import feature that ensures you are updating the correct profile. By using it in an import, you can update more data points, confident the correct profile is being updated each time. For more help with imports, check out the main article here.
If you have any questions, please don't hesitate to contact support.