RIB-IGS and RMS Webservices

When we are trying to add a supplier(through SOAP) into RMS 15 using RMS Webservice we are getting below error, also found in RMS operations guide that this is currently not supported

Whereas if we use RIB-IGS service , it is working fine.

Looking for below help

1 Any reason why this service is not implemented , ETA , also any differences in the implementation between RIB IGS vs RMS Webservices.

2. How much of an effort would it take create a web service client(for external system) to implement the IGS Vendor service

     Any sample code/details would be a great help.

Thanks in advance