What it means
Google's transfer tool can copy media to a supported outside service. A backup is a separately recoverable, tested copy that protects against source loss and can be restored when needed. A transfer may omit certain metadata or unsupported items and does not automatically synchronize future Google Photos changes. Completion of the transfer job is therefore only the beginning of validation.
A real-world example
A photographer moves years of images to another service and sees the total count match. Later they find comments and some manual metadata absent. A sample-based file and context check would have revealed the difference before deleting the original library.
What to do
Compare important albums, dates, file types, large videos and metadata in the destination. Keep the source until a separate backup and real restore test are complete.
The distinction that matters
The destination may have its own sharing and privacy defaults. A copy stored under another account can still be deleted or locked out. A successful transfer is neither proof of perfect preservation nor a replacement for ongoing backup. Check more than one year and file type: a large video, a recent phone photo, an edited image and an old album. Preserve the source until all of them are accessible and the new account's recovery route works.
Google Photos transfer limitations
