Any truth that you can give words to is a relative truth even if it is attempted to convey some understanding of Absolute Truth
Effectiveness is the measure of relative truth. To measure a relative truth you need a context. One and one makes two can be determined to be true it you are talking about basic arithmetic. However, as the child’s joke goes, one and one makes 11, if you referring to placement of two numeral 1’s next to each other. One and one can also make “one” if you are referring to the result of the union of two humans(or any other other animals which use sexual reproduction and with single offspring results) of the opposite sex (It can also be, more rarely, “two” in case of twins, “three” in the case of triplets etc). It can also be three (or more) if you include the parents in the equation. In each of these contexts, a different answer as to what one and one makes is true. You can’t say one answer is more true then the others except in specific contexts.
Therefore, Relative truths are always context specific. However not all contexts are equal in scope. Some relative Truths are true in such large scopes they have almost universal in application. Humans need water to live is true of all humans as we currently define them. Humans have brown skin is true but for a smaller context. Humans like death metal rock music is true but, (fortunately 
) for a much smaller contexts. If you added “all” in front of “human ” for each of the previous assertions then only the first one would be true.
Relative truths are true if they function within the contexts or domains to which they apply or belong. A systems, model or map is a relative truth that is a collections of relative truths connected to a specific function or set of functions. A system, model or map can be considered to be relatively true to the degree it is internally self consistent and fulfills the function or functions with which it is correlated.
For example modern science is concerned with understanding measurable phenomena. (As such modern science is not concerned with Absolute Truth at all because it is immeasurable.) Science as a whole is relatively true because, for the most part, it is internally self consistent and models or explains physical phenomena in a way that is functional. Individual postulates of science are true if they consistently correlate to all measurable phenomena currently known through observation. When phenomena is observed (repeatedly) that violates currently established postulates then, when science is functioning correctly according to its own internal self constancy, the postulates have to be revised to include the phenomena in its modeling. Science is relatively true to the degree that it successfully allows us to predict and/or effect in a predictable way measurable phenomena. Science becomes dysfunctional when its postulates are taken for absolute truths rather then revisable maps. Anytime science is used to declare something as impossible, it has become dysfunctional. In order to declare something impossible scientifically, everything about Universe must be known and measured and since Universe always has unknowable and immeasurable aspects never can everything about Universe be known. Science can, however, declare something as improbable according to currently limits of knowledge.
In the next post I will begin an in depth example of the function of Principle of Two Truths in a different system - Vajrayana Buddhism. This example will illustrate many of the subtle nuances of its functioning. For some I think this specific example will better convey the function of the Principle then trying to convey it general terms