All technical illustrators deal with projections to represent 3D objects on a plane. They apply standardized projections such as isometry and dimetry, or work in free projections and vanishing point perspectives. All techniques to compensate or at least minimize the drawbacks of representation on the plane and to convey 3D with only one representation.
Technical illustrations are supposed to convey content (messages). Graffiti should also convey messages (content). In a good technical illustration, the content can be understood by the addressee depending from the context. In graffiti it is rather difficult to get at the explicit or implicit message of the writer.
For those moments when we are not quite sure what our counterpart actually means something, we have a “credibility test” developed over generations at our disposal. When in doubt, we fall back on the oldest, most proven signal system, body language, then on the voice and last but not least we trust in the content of what is being said.