I need help with quadrotor_propulsion.cpp code

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Here's the code, I am trying to understand it, but I don't understand any of these variables, for example:

boost::array<real_T,4>  x;
boost::array<real_T,4>  x_pred;
boost::array<real_T,10> u;  (line 79, why do we have 10 here ?)
boost::array<real_T,14> y; (line 80 of the code, why do we have 14 here) ?

http://docs.ros.org/jade/api/hector_q...

I would really appreciate if you explained to me this, at least a little bit. I don't know on what dynamic model this code was based on, that is why I don't understand it.