I think history supports this, but I’m not sure I can cite a reference. Early Unix code I wrote followed the convention that codes 1-3 belong to system utilitie

I think history supports this, but I’m not sure I can cite a reference. Early Unix code I wrote followed the convention that codes 1-3 belong to system utilities, 4-9 are unreserved, and users pick 10-127. Some exotic use cases wrap some meaning into higher codes, but historically we only cared to detect whether the error comes from system binaries or within the application. I suppose programming on top of the application would mean using 20, but I still pick 10 by habit.