A front derailleur needs to have the right clamp diameter to fit your frame, cable routing (top or bottom pull) and swing (top or bottom). Most Shimano MTB derailleurs have multi-fit clamps and will operate with cable pulling from top or bottom. Top swing derailleurs have the clamp below the cage, bottom swing ones have the clamp at the top. Which of these you require depends on your frame so it's best to copy the one you already have.