Back when IR first started, and for the first 10 years or so thereafter, if you were shooting from an enclosed area while still having the bullet travel outdoors being subjected to all the elements and conditions, that would have been called MODIFIED rather than Indoor.
We ran a lot of Modified matches at Pinnacle Mountain during those winter's, and I believe some of those scores may be still Archived somewhere under the IR website. Then the Cook's decided to take that listing away and made doing so the same as OUTDOOR whenever it was only the shooter and gun that were inside and protected from the elements while shooting through portholes with the bullet still having to travel from the end of the barrel to the target, and INDOOR shooting then and now meant shooting at places like Piney Hill where you're in a specially built Indoor facility.
Dave Shattuck