Is there a rule on acronyms and initialisms being spelled out with the first letter of each word being uppercase or lowercase?
- interim final rule (IFR)
- Interim Final Rule (IFR)
The Chicago Manual of Style, sixteenth edition (2010) briefly addresses the question of whether the spelled-out form of an initialism or acronym should be initial-capped if the short form is capitalized, at 10.6 Capital versus lowercase for acronyms and initialisms:
On the other hand, if the words in a spelled-out version of an acronym or initialism are not derived from proper nouns or do not themselves constitute a proper noun (as in the official name of an organization), they should generally be lowercased, even when they appear alongside the abbreviated form.
[Example:] transmission-controlled protocol/Internet protocol (TCP/IP)
This is an accurate representation of standard U.S. publishing-house practice, in my experience, but there are lots of instances where at least some publishers seem to be influenced by the capitalization of the initialism to initial-cap the spelled-out form as well. Thus, for example, the first 50 matches that a Google search returns for the phrase "sports utility vehicle (SUV)," where the phrase appears in the course of a normal sentence (and not in, say, a title or subhead), include 38 instances of "sports utility vehicle (SUV)" and 12 instances of "Sports Utility Vehicle (SUV)."
Contrarily, the first 50 matches that a Google search returns for the phrase "interim final rule (IFR)," where the phrase appears in the course of a normal sentence, include 20 instances of "interim final rule (IFR)" and 30 instances of "Interim Final Rule (IFR)"—although in this case a few of the initial-capped matches may be parts of longer proper names, as in this sentence from the website Human Rights Watch:
Human Rights Watch unequivocally opposes the Interim Final Rule (IFR) on Moral Exemptions and Accommodations for the Coverage of Certain Preventative Services under the ACA (CMS-9925-IFC).
In any event, if "interim final rule (IFR)" is not part of a proper name, I see no reason to buck what Chicago calls the general convention to render the spelled-out form of the term in all-lowercase letters. But if you prefer to initial-cap the spelled-out term, you can find many examples online where other writers and publishers have chosen to do exactly that.