It's unclear to me what exactly fills my "57% of 15 GB used". I imagine attachments count, but is that both on attachments sent and attachments received? If I delete the last remnant of that attachment (either my sent copy or all copies received), does that free the space, or is it permanently part of my space as long as the recipient/sender holds it? If we both use Gmail, does it use space in both of our accounts?
Do uploaded (non-external) embedded images count similarly, with the same rules as above? Do uploads re-included in a forwarded email count doubly (i.e., once for the original message, again for every instance of reinclusion)? Is the space freed upon move to the trash or upon actual deletion a month later?
To clarify: I know that Google Drive shares the space—I'm interested in how Gmail handles these specific edge cases.