In the 1950s, the Billboard charts was divided up into different charts for all Pop singles. The best example of this is to view the charts in 1955. You will see each week had three different Top 20s (some went up to Top 30). One of those charts made up by Billboard was titled, “US Top 20 Most Played by Jockeys Singles”. This is the exact same principle as the “A” next to those songs in the 90s that were never released as a single. The only difference between the 1950s “Most Played By Jockeys” and “Airplay” singles, is back in the 50s those were actually released as physical records whereas the 90s were not.
In the 90s, those songs were taken from their parent album, or given to the radio stations in a demo CD form - some even B-Sides to physical CD singles. It was too confusing for Billboard, who finally gave up and just compiled all charts to make up the Hot 100 in 1958 (“Poor Little Fool” by Ricky Nelson was the first #1 on the Hot 100).
It’s been argued time and time again by those in the business whether an “Airplay” single is actually a single. The radio station I currently work for includes these as “Top Hit” standards - mostly because the owner loved, “Mr. Jones” by Counting Crows and decided it was ranked high enough to be considered a “hit single” in his eyes. However, you will not find “Mr. Jones” on the Billboard Hot 100 list.
I know it’s weird, but sometimes, some publications made the rules, whether we agreed with them or not.