MatterControl filters G-Code as it sends it to the printer via serial USB communication to maintain compatibility with a large range of printer types.
MatterControl accepts G91 commands for relative positioning, but instead of just allowing them straight through it interprets them as absolute coordinates and feeds those to the printer.
Starting in version 1.6, the Use G0 setting will allow for the use of G0 if desired.
To save bandwidth, MatterControl truncates superfluous coordinates in movement commands. For example, if the printer is currently at coordinates X30 Y40 Z50 and the next G-Code command is:
G1 X30 Y70 Z50
then MatterControl will truncate this command to:
since the X and Z coordinates are the same as the current position.