In the haystack tagging convention there are a lot of specific equipment refs like ahuRef, boilerPlantRef etc. Why is that? Why not just have one abstract equipRef that points to equipment, regardless of it's creed. You always have the ability to specify the type of equipment with the equip marker tags like ahu or boilerPlant. It seems a bit redundant and confusing to me. For instance when do you assign an equipRef and when a specific one?

Can one say that specific equipRef's are a sub class of equipRef and it's therefore left to the engineer to choose either a specific or the generic one?

Would someone please help me out in understanding this?

The equipRef tag is really for arbitrary "containment" relationships. Its more for navigation and organization, then semantics. The others define explicit semantic relationships. For example an ahu might have a chillerPlantRef and a boilerPlantRef which represent the supply of chilled/hot water. You couldn't do that with a single equipRef tag.