The MMC-100 doesn't provide for tape deck synchronization to a sequencer nor from one tape deck to another.
MMC (MIDI Machine Control) allows a software or hardware controller to control basic transport functions of another device like Play, Stop, Pause, Record, as well as Locate. In this case the MMC-100 was designed for certain Tascam products that have the Accessory 2 port... 238, TSR-8, MSR-16, etc.
The MMC-100 does however convert SMPTE to MTC (MIDI Time Code) which allows a sequencer to slave to a tape deck when SMPTE is striped to one track of the ATR and read by the MMC-100. You could use it that way without using the MIDI Machine Control functionality of the Accessory 2 port. You can also use the remote control function via the Accessory 2 port without using the sync feature.
So basically the MMC-100 can be seen like any SMPTE-to-MTC box (JL Cooper, Philip Rees etc), but with the addition of MMC to allow remote control of ATR transport functions. The device generating the MMC commands could be a computer or a MIDI keyboard controller.
If using both features together you can start the ATR from a computer and then the sync track on the ATR sends SMPTE to the MMC-100, which converts it to MTC and sends it to a sequencer, which slaves to the Tascam ATR.