What I have found to work best is start by filtering your existing inventory, assuming you have at least one fact sheet of the type you eventually want to import. That provides a point of reference template and some example data. Then: 

1. Select the table view.
2. Select the “select columns” icon.
3. Add any columns to your view that you will later want to import, such as subtype.
4. Export that to use as your import template.
5. Update that exported spreadsheet to turn it into your import spreadsheet so that upon import it will add, change, or delete fact sheets.
    

See: https://docs-eam.leanix.net/docs/import-export-your-data-via-excel

I am a rookie and working with a mostly standard, out-of-the-box configuration that was recently provisioned. If you follow the above and do not see subtype as an option, your configuration may be different.

@Mark Arend great post and is certainly true of workspace configurations started at or converted to Meta-Model v4.

Workspaces with earlier versions of the meta-model may need to follow the steps outlined here Configure Workspace for Meta Model v4 (leanix.net) and the subtype may be called “category”.

@Mark Arend That is really helpful! I just updated all subtypes in my workplace. 

Thank you for your detailed explanation! 
Sofia

Thank you @Mark Arend for sharing your best practice!