The Mainstream Top 40 airplay-based chart debuted in Billboard magazine in its issue dated October 3, 1992, with rankings determined by monitored airplay from data compiled by Broadcast Data Systems, a then-new technology which can detect when and how often songs are being played on radio stations.
[1] Mainstream Top 40 is compiled from airplay on radio stations which play a wide variety of music, not just "pure pop", which Billboard defines as "melodic, often synth-driven, uptempo fare".
[2] During the 1990s, mainstream top 40 went from R&B dominating the airwaves (and thus the charts) in the early 1990s to rock and alternative music becoming the choice of program directors in the latter part of the decade.
[2] The mid-1990s also witnessed a drastic difference between what reached the top of the Mainstream Top 40 chart and the Hot 100, when songs started being promoted to radio and receiving significant airplay without the release of a commercially available single, a requirement for a song to reach the Hot 100.
Thus, number-one songs on the Mainstream Top 40 such as "I'll Be There for You", "Fly", "Don't Speak", "Lovefool", "Torn", "Uninvited", and "Iris" failed to reach or have a similar impact on the Hot 100.