The discussion identifies eight vocative particles. Their original distribution distinguishes a nearby addressee from one at a distance.
ParticleBasic use
أَ، أَيْNear addressee
يَا، آ، آيْOriginally for a distant addressee
أَيَا، هَيَا، وَاOriginally for a distant addressee
This is a classification by original linguistic assignment. Rhetorical usage can treat a remote person as near or a near person as distant. Wā is also familiar in the special form of lamentation (nudbah).