This is just like nsf, but allows the use of sound expansions, mainly through the use of mappers. A mapper is an integrated circuit inside of a cartridge that allows for more sophisticated operations to be performed on the Famicom/NES, including memory management and additional sound. A mapper would be chosen depending on the size, needs and/or resources, or none at all as was the case with Super Mario Bros.
Mappers that offered/expressed additional sound functionality were found only in Japanese cartridges, mainly because they easily implemented because of the Famicom's pin configuration on cartridges. In other regions, the pins for additional sound are found on the rarely used expansion port located on the bottom of the original console (top-loaders omit this expansion port).
These chips were manufactured by Konami, Namco, and even Nintendo themselves. The Famicom Disk System (FDS) from Nintendo, which plugged into the cartridge slot, was different in that it contained a dedicated chip, the 2C33, that added a pseudo-FM channel to the Famicom.
Many programs that spin out NSFs allow for multiple mappers to be used at once (mainly MCKs at the time of writing). However, it must be noted that implementing multiple mappers on a cart would be a difficult (or perhaps impossible) task. Only one mapper was used at a time on a cart as there was no need to be so insane. In addition, using the FDS sound expansion already creates a challenge with using multiple chips at once because there is no longer a small board to tackle, but a machine.
It is recommended that the bitpack for an OHC specify which expansion chip to use for an NSF+ compo, unless there is some other strong theme to keep the entries coherent.