Ciphers-SUITEB128ONLY is generally an OpenSSL Cipher Suite setting that enables suite B mode operation using 128 (NOT permitting SUITEB192) 128-bit level of security.

There is no ongoing Standards definition for Ciphers-SUITEB128ONLY but is is general "short-cut" used within OpenSSL and others for setting a group Cipher Suite settings.
RFC 8423 titled: Reclassification of Suite B Documents to Historic Status

If used these cipherstrings should appear first in the cipher list and anything after them is ignored.

Setting Suite B mode has additional consequences required to comply with RFC 6460. In particular the supported Digital Signature algorithms is reduced to support only ECDSA and SHA256 or SHA384, only the Elliptic Curve P-256 and P-384 can be used and only the two suite B compliant Cipher Suites (ECDHE-ECDSA-AES128-GCM-SHA256 and ECDHE-ECDSA-AES256-GCM-SHA384) are permissible.

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