Does the Photos app use the iPhotos Library file?

I'm trying to delete unneeded files from my MacBook Air to free up space, and there's a file taking up 70GB called "iPhotos Library." The system identifies it as a "Migrated Photo Library," but it isn't clear if this means the data have been duplicated and moved to a new file or if the library itself has been converted and is in use by the newer Photos app. Trying to open the file brings up this error message, and clicking the More Info button takes me to a support page that's not informative.

I obviously don't want to delete all my old photos, but if it's "Migrated" does this mean that the photos have been imported to a different file, so I can delete the old one?