filler vs written matter

filler and written matter both are nouns.

Word	Noun	Adjective	Verb	Adverb
filler	Yes	No	No	No
written matter	Yes	No	No	No

As nouns, written matter is a hypernym of filler; that is, written matter is a word with a broader meaning than filler:

filler: copy to fill space between more important articles in the layout of a magazine or newspaper
written matter: matter to be printed; exclusive of graphical materials

Other hypernyms of filler include copy.

filler (noun)	written matter (noun)
the tobacco used to form the core of a cigar	matter to be printed; exclusive of graphical materials
anything added to fill out a whole
copy to fill space between more important articles in the layout of a magazine or newspaper
100 filler equal 1 forint in Hungary
used for filling cracks or holes in a surface