Rcm case in gst law

Can any one give the RCM with examples , it is very complicated me. RCM will be applicable for revenue expenditure in the business. If trader has paid rent to the land lord for services through bank, the land lord has not registered in gst and he was not registered in the service tax also on the earlier. But he has a assessee . In this situation when we pay the rent to him for business purpose, the reversal charges can be applicable or not. (2) RCM is applicable for expenses. can give the brief introduction of this topic in a simple manner.