TLS_Ciphersuites specifies which subset of TLSv1.3 cipher suites is permissible for a connection. The parameter format is documented in OpenSSL manual for SSL_CTX_set_ciphersuites() API function. For example: TLS_AES_256_GCM_SHA384:TLS_CHACHA20_POLY1305_SHA256. Usually there is no need to set this parameter. There is no default and the system default is used.

# This cipher suite is mandatory for TLSv1.3
TLS_Ciphersuites TLS_AES_128_GCM_SHA256 

This parameter affects only TLSv1.3. For the respective parameter for TLSv1.2 and earlier TLS versions versions, see Section 3.11.9. TLS_Ciphers