you should refer this link https://www.google.co.in/url?sa=t&source=web&rct=j&url=https://www.cbec.gov.in/htdocs-cbec/gst/reverse-charge-mechanism-08aug2017.pdf&ved=2ahUKEwimr6uo-b7ZAhWFlJQKHaBPCEAQFjACegQIBBAB&usg=AOvVaw1HgB2ToagUE1hMpi84O6xN
Here all your queries will be resolved. this pdf have a great summary type explanation.