DKIM d= tag
The tag in a DKIM signature naming the signing domain, which takes responsibility for the message and whose DNS holds the public key.
What it means
d= is the Signing Domain Identifier (SDID). Together with s= it locates the public key (s._domainkey.d), and it is the identity a receiver attaches reputation to. A valid signature says that domain takes responsibility for the message; it says nothing yet about the From address.
DMARC uses d= for alignment. With relaxed alignment (the default), d=mail.example.com aligns with a From address at example.com because both share the organizational domain. With adkim=s, the two must be identical.
A common misconfiguration: a sending platform signs with its own domain (d=platform.example) instead of yours. DKIM passes, but it does not align, so DMARC fails unless SPF aligns. Setting up custom DKIM for your domain on the platform fixes it.
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