This is the best example as to the root of coordinate systems with in 3d space.
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A bit silly but is typical of most of not all 3d applications as to there not being an accepted and established coordinate system as being referenced by X,Y,Z. In most cases the idea of direction is based on the requirements of a given project as determined by an individual, project leader, by first determining what is to be the point of reference. Also in a lot of cases the application of choice helps you out by establishing a “default” system that works with the tools they provide “relative” to their established world space using world origin and units as to base function such as rotation velocity and transformation.
Units for example is not a true measurement but a means to established a form of measurement that is true to all applications that once again the end user can determine what the value represents. In UE4 for example world space = 1unit =1cm so accepting world origin as 0,0,0 and 1unit = 1cm more complex math could be worked out with out having to resort to what is being provided. Kind of reminds me of the math mumbo jumbo used in the movie “Cube” :D.
However
UE4 helps you out in some areas as to common use by providing tools that already define a practical coordinate system built in. The most typical is the character blueprint which by default provides a forward vector along with a way to determine things like velocity ,transformation and rotation. Just need to keep in mind that just because it’s called a character blueprint does not mean you can only make use of it using characters.
This of course only applies to the use of world space.