Could please somebody explain, in which cases pages should be linked to this category? Isn't every discussion of people with different opinions who try to come to a consensus a negotiation?
You are right, but I think the pages that have this category are about formal negotiating sessions, where the outcome is some form of a signed, binding contract. Examples are employment contracts, contracting contracts ?!?, treaties, etc.
A useful starting point for this category is the NegotiatingPatternLanguage.