Relevant Products: Exclaimer Cloud - Signatures for Office 365
You've enabled the Outlook Web Signatures functionality, or you are thinking of doing it; you have multiple signatures configured to your Outlook Web mail account. Now, you are thinking which signature will show when you draft an email.
Outlook Web can only configure one signature. If you have multiple signatures, then the first signature is treated as the default signature and will be applied to the email.
In case you make any changes to a signature, then the new signature will overwrite the old signature.