The 8 tracks on the MMT-8 come together to compose an entire part. These parts are then numbered from 0-99 and can be connected together in sequence to create a song.
To erase a part (which erases all 8 tracks that make up said part) do these things.
Turn on the Part button.
Select the part you want to erase using the + and - buttons.
Hold down the erase button and then push the record button.