In case anyone else wants to know the answer, this is the reply i got from support:
The "mul" means mutiplicator.
Midi use's a range of value of 128 and DMX use's 256.
The mul is set to 2 to convert the midi value into DMX value.
In some case's the midi range could be 64 in this case you can set the mul to 4
For your information there is also a "mul" In the fixture Module :
Because the most of the keywords in modul8 need a value from 0 to 1.
By default the mul is 1 and it mean the full value in DMX will be convert to 1.
Some keywords (but it's rare) need other range of values, for exemple :
"ctrl_ui_previewWindowMode" need's a value from 0 to 6
In this case you can set the mul to 6.
Thank you Boris. =)