I have the same machine and also had to purchase the walking foot separately. I called customer service at Allbrands.com, told the guy which machine I have and what foot I wanted, and they sent it to me at an affordable price. I was unsure that I was getting the correct foot, and he told me if I was unhappy with it, I could return it. Fortunately it worked perfectly and was exactly what I wanted.
I think what you may be asking is what's the difference between a high-shank machine and a low-shank machine? It's the height of the attachment. The walking foot for my SE400 will not fit on my Brother 1500 because one os a low shank and the other is a high shank.