What's this option in the AOSPA stock email app?
I believe your assumption is exactly correct.

Google has laid out these settings the same way as they have in Gmail, which has better translations. The bottom one is indeed the per-message notification toggle.

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You could also try temporarily changing your language to English to see if the description is any better thay way. Or, of course, you could send yourself a couple e-mails with and without the setting checked and see how the notifications behave.