With both the WBC and the WBA based in Latin America by the late '70's, the USBA was formed in 1976 as a sanctioning body comprised of 24 American boxing commissions. In 1983, the USBA decided to go international, forming a new division called the IBF, which eventually subsumed the USBA. The IBF was able to receive almost immediate recognition as a major sanctioning body when Larry Holmes chose to dump his WBC title in favor of the IBF's title. To this day, the USBA remains a part of the IBF, and because of its setup, you're a bit more likely to see American ranked challengers in the IBF than the other sanctioning bodies. However, the IBF is not recognized as a major title in all parts of the world, as Japan still does not recognize the IBF as a legitimate sanctioning body.
Despite being one of the newer bodies, the IBF has not avoided controversy. In 1999, then-IBF president Robert W. Lee was indicted on racketeering charges, accused of having accepted bribes to rig their rankings. Unlike the WBC, who kept leadership in place after their scandal, the IBF seemed to have learned from its mistakes, instituting a slightly more public ranking process and getting rid of the bad apples from the cart. For about five years, the IBF was actually monitored by the Federal government to make sure it was a legitimate organization and not a function of organized crime.
One good thing about the IBF is that it has managed to avoid having multiple belts like the WBA and the WBC. The IBF champion is the IBF champion, and that's it. No super champions, no interim champions, no champions emeritus, no champions of the universe. Instead, it has a somewhat convoluted system of becoming a mandatory challenger, although this system usually helps ensure that the challenger is a worthy one. The ratings committee determines rankings of the fighters in the 3 - 15 slots much like any other sanctioning body, but the #1 and #2 challenger spots are left blank. The IBF then generally has title eliminators to fill both the #1 challenger and #2 challenger spots. Once those two slots are filled, the #1 and the #2 will usually fight to become the mandatory. The end effect is that a mandatory challenger generally has earned that right by defeating at least two other highly ranked fighters to earn the shot.
No WBA or WBA titlist can be ranked in the IBF ratings, although the IBF doesn't recognize the WBO as a major title and will allow the WBO titlist to be ranked. To remain ranked in the top 10, a fighter must face at least one other person ranked in the top 15 every 18 months. Also, in addition to being required to face mandatories, voluntary defenses for champions generally must be against fighters ranked in the IBF's top 10. Within the top 15, there is a pretty stringent set of rules as to how a fighter moves up or down. If a ranked fighter loses to an unranked fighter, he drops out of the rankings completely. If two ranked fighters face each other and the higher ranked fighter wins, he moves up, and if the lower ranked fighter wins, the fighters switch spots in the rankings.
There is, however, a downside to this system. First, it makes it very difficult to unify the IBF title. The benefit of having a super champion is that a champ can wait longer between title defenses. To get a unification match, the fighter would need to get an exception from the ratings committee and then still only make fights against top 10 rated IBF fighters. If the other body's mandatory isn't ranked in the IBF's top 10 (which it almost never will be, since challengers for other titles are ineligible to be ranked), then the champ would be forced to vacate one of the two titles.
Second, it means the rankings can look way off, especially when the top fighters in a weight class are all fighting each other. Take a look at welterweight, for instance. Right now, literally none of the better welterweights are ranked by the IBF. The top guys are fighting each other. That means the losers of those fights aren't eligible to be ranked, and the title challengers aren't eligible to be ranked. So what you end up with is a set of rankings where the top five is comprised of Rafal Jackiewicz, Randall Bailey, Delvin Rodriguez, Isaac Hlathswayo and Luis Carlos Abregu (who will probably drop out because of his loss to 'unranked' Timothy Bradley).