It has lost a lot of its meaning over the years. High pressure fuel systems can be a gray area where a manufacturer might not list a filter used as complying to their minimum standards. Nothing in their sales contract says you have to use their filter, but again nothing in their their warranty contract says they have to warranty a failure caused by some one else's parts. Here are a couple of statements from the law.
(c) Prohibition on conditions for written or implied warranty; waiver by Commission
No warrantor of a consumer product may condition his written or implied warranty of such product on the consumer's using, in connection with such product, any article or service (other than article or service provided without charge under the terms of the warranty) which is identified by brand, trade, or corporate name; except that the prohibition of this subsection may be waived by the Commission if—
(1) the warrantor satisfies the Commission that the warranted product will function properly only if the article or service so identified is used in connection with the warranted product, and
(2) the Commission finds that such a waiver is in the public interest.
(c) Waiver of standards
The performance of the duties under subsection (a) shall not be required of the warrantor if he can show that the defect, malfunction, or failure of any warranted consumer product to conform with a written warranty, was caused by damage (not resulting from defect or malfunction) while in the possession of the consumer, or unreasonable use (including failure to provide reasonable and necessary maintenance).
The entire law can be found at the following link:
https://uscode.house.gov/view.xhtml?req=granuleid:USC-prelim-title15-chapter50&edition=prelim
