Each machine assembly that is created must have an custom MDD that defines that machine including the tool change position, the range of motion of the axes and other data. In the case of many fairly simple machines you can start with an existing, standard MDD and modify the data to fit the machine. Whenever the MDD is selected in the Document Control dialog, the accompanying machine assembly file will be the default machine for Machine Sim. The MDD should be reviewed after you have created the machine assembly file. It is important that the MDD and machine assembly file be consistent in terms of axes definition. This includes number and type of axes, the axes position, orientation and order. See the MDD Editor documentation for information on creating an MDD.
In addition to the default machine, the MDD for a Machine Simulation model can set other information. For example, the rotary information for a 4 or 5-axis machine will be pre-determined, meaning that it does not need to be set each time a part is created. The tool change position may be pre-set, depending on your machine and preference. Also, the default post processor is set by the Machine Sim MDD.