Tesla’s “Autopilot” system can do some impressive things.

It can steer a car through traffic and execute passing maneuvers with no involvement from the driver other than a flick of a turn-signal stalk. But while it does enable some autonomous driving, Autopilot does not turn a Tesla Model S or Model X into a self-driving car.

Autopilot’s capabilities are limited, and Tesla was quick to emphasize those limitations when it released the system in “public beta” last October. At the time, Tesla CEO Elon Musk said Autopilot worked most effectively in dense traffic, on roads with very clear lane markings.

Tesla Autopilot suite of features - with version 7.0 updateEven in a best-case scenario, though, he advised drivers to keep their hands on the wheel and stay alert at all times. He also noted that Autopilot’s capabilities were reduced by rain or snow.

Aside from its functional limits, Autopilot is also missing a feature crucial to self-driving cars. It isn’t connected to a car’s navigation system, so it can’t simply follow a route while the driver takes a nap, notes a recent Forbes review of the tech.

That puts Autopilot very low on the scale of vehicle autonomy, as defined by the U.S. National Highway Traffic Safety Administration (NHTSA). The scale includes five levels of autonomy and seeks to clarify the difference between fully autonomous cars and vehicles with some autonomous capabilities.

Level 0 means a car has no autonomous systems, to speak of, or advanced driver aids that intervene to provide assistance.

Tesla Autopilot sensor systemLevel 1 encompasses cars with advanced driver aids, including electronic stability control and systems that pre-charge the brakes in anticipation of a collision. These assist the driver but do not take over control of the vehicle.

A Level 2 vehicle features automation of at least two control functions that work together, allowing the driver to cede limited control.

(With its ability to maintain a set distance from a car ahead and keep a car centered in its lane, Autopilot seems to fall into this category.)

Level 3 autonomy indicates a car that would be able to follow a route and drive itself without any human involvement, albeit only in certain situations. Cars with Level 3 autonomy still require a human driver on board ready to take over.

Level 4 defines a car that can drive itself over an entire trip, with a human occupant only selecting a destination.

2016 Tesla Model SMusk believes Tesla can achieve this final stage of autonomy eventually, and, in the meantime, the company is working to improve Autopilot. Engineers currently download data from customer cars to see how the system behaves in the real world. They also use special software to run simulations of new Autopilot features on customers’ cars as those cars travel down the road.

This story originally appeared on Green Car Reports. Copyright 2016