The non-desperate search through another ill-documented project with good examples
ACADOS uses some strange pre-processor directives to enumerate indices used for the size of state and control vectors. They can be located in acados_solver_<your_project_name>.h. Here is what they mean:
QUADROTOR_ODE_NX : This is the size/length of the state vector of the model.
QUADROTOR_ODE_NY0 : We use yref to set elements of the cost function, and this is the length of yref for zeroth iteration which is the initial state.
QUADROTOR_ODE_NY : Length of yref for iterations from 1 upto N-1.
QUADROTOR_ODE_NYN : Length of yref for the last iteraton N, is smaller than QUADROTOR_ODE_NY by the size of control vector(QUADROTOR_ODE_NU) because control reference is not provided for the last shooting node.
QUADROTOR_ODE_N : Number of shooting nodes / number of predictions.
QUADROTOR_ODE_NU : Length of the control input vector.
QUADROTOR_ODE_NP : Length of the parameters vector supplied to acados.