The "MIC-ONLY" specifier signifies that all of the security services specified for ENCRYPTED messages, with the exception of confidentiality, have been applied to a PEM message's encapsulated text. MIC-ONLY messages are encoded (per Section 188.8.131.52 of this RFC) to protect their encapsulated text against modifications at message transfer or relay points.
Specification of MIC-ONLY, when applied in conjunction with certain combinations of key management and MIC algorithm options, permits certain fields which are superfluous in the absence of encryption to be omitted from the encapsulated header. In particular, when a keyless MIC computation is employed for recipients for whom asymmetric cryptography is used, "Recipient-ID-Asymmetric:" and "Key-Info:" fields can be omitted. The "DEK-Info:" field can be omitted for all "MIC-ONLY" messages.