standards – What is the PKCS#7 detached signature format?

This website claims that (emphasis added):

In PKCS#7 SignedData, attached and detached formats are supported… In detached format, data that is signed is not embedded inside the SignedData package instead it is placed at some external location…

However, RFC2315 seems to define no such “detached” format.

  • SignedData comprises:
    • version = INTEGER 1
    • digestAlgorithms: SET OF DigestAlgorithmIdentifier
    • contentInfo, comprising:
      • contentType = data (OBJECT ID “1.2.840.113549.1.7.1”)
      • content: Data = the message that was signed
    • signerInfos: SET OF SignerInfo, each comprising:
      • version = INTEGER 1
      • issuerAndSerialNumber
      • digestAlgorithm: DigestAlgorithmIdentifier
      • encryptedDigest: EncryptedDigest = the low-level signature

Neither SignedData/contentInfo nor ContentInfo/content:data are OPTIONAL; and the former may only be one of the six defined types: data, signedData, envelopedData, signedAndEnvelopedData, digestedData, and encryptedData; I see no detachedData or any similar options. (Choosing digestedData does not resolve it, as ContentInfo/content:digestedData is also non-optional.)

What belongs in the contentInfo field of a detached SignedData object?