This reports the size in bits of the key used for encrypting payloads on this specific tunnel, per its description. It's the concrete strength measurement paired with cisgIpsSgTunEncryptAlgo - the same algorithm name can correspond to different key sizes, so this object tells an admin exactly which variant is in use on this tunnel. A tunnel reporting AES as its algorithm but only 128 here, when the security policy calls for AES-256 everywhere, flags a specific tunnel not meeting the intended standard.