subhead vs heading

subhead and heading both are nouns.

Word	Noun	Adjective	Verb	Adverb
subhead	Yes	No	No	No
heading	Yes	No	No	No

As nouns, heading is a hypernym of subhead; that is, heading is a word with a broader meaning than subhead:

subhead: a heading of a subdivision of a text
heading: a line of text serving to indicate what the passage below it is about

Other hypernyms of subhead include head, header.

subhead (noun)	heading (noun)
a heading of a subdivision of a text	a horizontal (or nearly horizontal) passageway in a mine
	a line of text serving to indicate what the passage below it is about
	the direction or path along which something moves or along which it lies