It is one of the command line parameters when starting the XAS. From memory, it goes like this:
java -jar mx.jar -m P -f "path/to/model.mdp" -o "path/to/application.conf".
The value of the
-m parameter (D, T, A or P) defines the DTAP mode, specify the model file with the
-f parameter and the configuration file(s)** with the
** You can use multiple
-o parameters (the configuration files will be parsed in the order from left to right), useful for "overloading" specific configuration elements or just for splitting up the configuration in multiple files.
Edit: there is more information on the Mendix wiki here.
When using DTAP options, also look at the description about different behaviour (draft) of the XAS in different modes.
As another example, here's the xas2 shell-script which Mendix Project Employees are actually using as helper tool when deploying projects in a Linux based server setup.