Hmm, are you referring to https://wiki.openstreetmap.org/wiki/Tag:parking%3Dstreet_side#Relation_to_area:highway=* ? I can't find any other examples for this.

Normally, parking doesn't need any other area-based tag. `area:highway` describes the physical contour/layout of the road, but this normally excludes parking, sidewalks, traffic islands, etc. - just the road surface itself.

I see there are some rare uses of `area:highway=parking_space`. But I don't think there is any need to add that here if the area of parking matches the area of surface exactly. That would be more useful for parking lots which include other elements, like grass medians and footways to indicate the exact parking area surface.