These commands are used when you want to have a variable in the "channel" (data1) parameter field.
Variables for the DMX Machine are only allowed to go up to 255. So, "DMXRamp256" handles channels 1-256, while "DXMRamp512" handles channels 257-512.
In other words, when your variable's value is "255" for the DMX Channel parameter, and "DMXRamp256" is used, the channel that will be changed is DMX Channel 256. When your variable's value is "255" and "DMXRamp512" is used, the channel that will be changed is 512. Similarly, if "DMXRamp256" is used and your variable's value is "0", the channel that will be changed is DMX Channel 1. If "DMXRamp512" is used and your variable's value is "0", the DMX Channel that will be change is 257.