postmark vs marking

postmark and marking both are nouns.

Word	Noun	Adjective	Verb	Adverb
postmark	Yes	No	Yes	No
marking	Yes	No	No	No

As nouns, marking is a hypernym of postmark; that is, marking is a word with a broader meaning than postmark:

postmark: a cancellation mark stamped on mail by postal officials; indicates the post office and date of mailing
marking: a distinguishing symbol

Other hypernyms of postmark include mark, marker.

postmark (noun)	marking (noun)
a cancellation mark stamped on mail by postal officials; indicates the post office and date of mailing	evaluation of performance by assigning a grade or score
	the act of making a visible mark on a surface
	a pattern of marks
	a distinguishing symbol

postmark (verb)	marking (verb)
stamp with a postmark to indicate date and time of mailing