MED-Part-B Request and Response FHIR Standards

We need some help on what FHIR standards we need to implement for inbound and outbound request/response . For MED Part B service our vendor do a POST request to us (XML over https) and we send them response with status code & message along with one more field.

For Request inbound to us and request outbound to vendor we looking for FHIR standard format, also for replying vendor with acknowledgement.

Can you explain what “MED Part B” is?