What does "attach to content type" option in Feeds importer do?
Yeah - I found this confusing as well.

Personally, I always select "standalone" which means that when I'm ready to import nodes with an importer I've created, I go to the path /import (e.g., http://example.com/import)

On this screen, I can select my importer and then launch the actual import (the one I created under /structure/feeds).

If you decide to attach to a content type instead, it just means that you go to /content/add/[content type] to launch the import process rather than /import. This way just never made sense to me conceptually, but there are probably good reasons to use this approach. Probably depends on who (which roles) you want to be able to use feeds imports.

Anne