The default state depends on the FilterInitializationFcn property, which is initcvekf. initcvekf initializes a constant velocity (CV) extended Kalman filter (EKF). You can choose from a variety of other init functions by using tab-completion in the MATLAB Window:
This will give you a wide range of initialization functions. One of them, initcaekf, initializes filters based on the constant acceleration (CA) extended Kalman filter (EKF).
Please use the documentation of each function to know how the state vector is defined and which motion and measurement models are used.
Additionally, You can always create your own initalization functions, and in each one of them you can define the filter to use and which motion and measurement models to use. Please do not feel restricted to use the built-in motion and measurement models or the initialization functions that ship with the product.