attachment host VS resource_subtype

On that link of the documentation:

I see that “hosts” can be one of:
asana, dropbox, gdrive and box

And, resource_subtype one of:
asana, dropbox, gdrive, onedrive, box, and external

I suppose I should use “resource_subtype” instead of hosts, but what’s the difference with “onedrive” and “external” to exclude them from hosts?