How does public relations and inter-personal skills relate?which one encompasses the other?

in my view its the same like having the manners of eating at home or a restaurant. People are supposed to be well mannered while eating out. If they arent well mannered at their homes how are they supposed to follow the same manners while eating out. Same applies to inter-personal skills or public relations. People having the skills to communicate with their colleagues would be more useful while communicating with the people who are part of their business but not their colleagues.

I hope i have made my point clear.