"LDC" = Long Dust Cover. It refers to the frame's dust cover (the front/muzzle portion of it... it acts as a cover to keep dust out of the internals.)
Here is an example photo of the differences:
The top version is standard Marui Hi-Capa 5.1, with a standard "short" dust cover type frame, with scalloping in the muzzle of the slide to mate with the end of the short dust cover.
With an "LDC" setup (like the 4.3 and the 5" on the bottom), you'll notice that besides the longer dust cover on the frame, there are also no scalloped cuts in the slide. It is what we call "block cut" to mate with an LDC frame.
Ultimately, it is up to you to decide how you want to configure it... some people like the look of a block cut slide on a short dust cover frame, or the look of a scalloped slide on a long dust cover frame, but purists will scream bloody murder if you do such a thing. There ARE real steel manufacturers that mix it up on occasion, but it's generally not preferred. Mixing a block cut slide on to a short dust cover frame can cause damage to the bottom corners of the slide at the muzzle and/or can damage your holster as well as create difficulty in holstering the pistol. This is why scalloping exists.
On the flip side, putting a scalloped slide on a long dust cover frame can build up dust/dirt/grit around the scalloping where it meets the frame. This would tend to be counterproductive to the purpose of a dust cover.