To support upgrading to replica-set config servers (csrs) from existing deployments of sync cluster config servers (sccc), we must add a new configuration directive, sharding.configsvrMode and command line option, --configsvrMode=sccc. This directive is only legal on mongod and only if the --configsvr directive has been set.
Servers started with --configsvr and --confisvrMode=sccc act as an SCCC config server rather than a CSRS config server, even if the node is a member of a replica set. This flag’s only practical effect is to affect the response the node sends during ismaster handshake.
"sccc" is the only viable argument to --configsvrMode. --configsvrMode=csrs is implied by starting with --configSvr and --replSet. This flag allows suppression of that implied behavior during the CSRS upgrade process.