Webots PRO and Webots EDU allow users to create their own robot models with a customized shape, appearance, texture, mass distribution, sensors, actuators and controller programs. Robots can be programmed using any programming language (C/C++, Matlab, Python, Java, Choregraphe, Aseba, etc.). Similarly, they can also create custom objects that will interact with the simulated robots. There is no limit to the number of robots or objects a simulation can held.
Webots MOD doesn't allow users to create custom robots or objects. Instead, users should purchase the robots and objects they need. Each purchased robot is tied to a specific programming language (for example C/C++, Matlab, Python, Java, Choregraphe, Aseba, etc.) which can be used to program the robot. Several instances of a robot or an object should be purchased if the user needs to run a simulation containing them. For example, if you want to run a simulation of 2 NAO robots wrestling against each other, which you can program in C++, you will need to purchase two licenses of Webots for NAO C++.