Hi there,
Say if the prior ID sync took place with the CRM ID, and there were 2 devices linked to the single CRM ID.
Then when you will upload the offline file containing the CRM ID<signals>, then both the linked devices will be qualified for that trait, because of the CRM ID. You will see trait's device population as 2.
So, the trait qualification gets associated with the CRM ID or profile, and an ID sync should take place either before uploading the file or after uploading the file.
Moreover, while creating the segment using this trait, you will have to select a Profile Merge rule if the data source's type was cross device. Then using Profile Merge rule, you can select few options like authenticated + current device profile Or authenticated + Profile link device graph (gets device info from last 3 linked devices and current device).
Thanks,
Varun Kalra