When trying to understand the import format I always recommend the same procedure: create 3/4 records manually with different values and export them. If you already have comments in the system, select 4/5 and export them. The format you get is the format the application expects when importing.
To answer your question, in the export you should see something like this in the "related to" field:
Yes, with this tip I was able to load for service contracts
I put it as assigned to a user but even with the public profile the system does not display this users comments
It was created as created by the administrator (although in charge I point to this same user)
Two things
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1) In import I forced setting to my user ( in spreadsheet column has no value, only a header ), however the system put all comment as admin user ( was logged) in as creator
2) The user assigned to this is correct, but he does not see the comments ( mapped in import with assigned to )
I just imported the attached file to ServiceContracts with no problem at all. Similar to the video I added two related to a new SC and even added a new line to an existing SC.