What are pragmatic rules?

linguistic rules that help the communicators understand how messages can be used and interpreted in a given context.

"some pragmatic rules aren't shared by most people in a culture. In the US and Canada, for instance, competent communicators understand that the question "how's it going?" usually isn't really a request for information. Anyone familiar with the rules of conversation knows that the proper answer is something like "pretty good. how's it going with you?"