What is the purpose of category_id in provider schema?

[joffinjoy]

As per the latest core-spec schema, provider schema has got a field called category_id as shown below:

{
	"catalog": {
		"descriptor": {},
		"providers": [{
			"category_id": <String>,
		}],
		"fulfillments": []
	}
}

What is the purpose of this category_id? Placement of this field in providers hint at "a provider catering to a particular category", but isn't category a item level parameter? Like, a provider can provide multiple items, and each of those items can belong to multiple categories.

I am assuming this category_id is related to something else other than the item categories.

Probably related: #335

cc: @ravi-prakash-v, @aks30, @vijiurs, @kiranharidas187