The string defined by user to fill in the circuit ID
suboption of option 82 in the user-define mode.
If the value of hh3cDhcpRelay2IfOpt82CIDMode is NOT 'userDefine',
it is set to a zero-length string automatically.
Otherwise it must be set with a non-zero length string.
The SET operation of this object must be with the SET
operation of hh3cDhcpRelay2IfOpt82CIDMode.