tax write-off vs write-down

tax write-off and write-down both are nouns.

Word	Noun	Adjective	Verb	Adverb
tax write-off	Yes	No	No	No
write-down	Yes	No	No	No

As nouns, write-down is a hypernym of tax write-off; that is, write-down is a word with a broader meaning than tax write-off:

tax write-off: a reduction in the gross amount on which a tax is calculated; reduces taxes by the percentage fixed for the taxpayer's income bracket
write-down: (accounting) reduction in the book value of an asset

Other hypernyms of tax write-off include write-off.

tax write-off (noun)	write-down (noun)
a reduction in the gross amount on which a tax is calculated; reduces taxes by the percentage fixed for the taxpayer's income bracket	(accounting) reduction in the book value of an asset