Subaru will unveil a concept version of the next-generation Impreza in Tokyo later this month. it released this illustration on Wednesday. Image source: Fuji Heavy Industries.

The next-generation Subaru Impreza is an extremely important car for Subaru and its parent company, Fuji Heavy Industries (NASDAQOTH:FUJHY).

It's not just an all-new version of a strong-selling model. It's the first Subaru that will be built using a completely different approach to car design and production -- one that is expected to improve quality and save a lot of money at the same time.

The new Impreza will be a 2017 model. It won't arrive at U.S. dealers until late next year. But Subaru is gearing up to give us a preview -- along with a deeper look at where the company is heading.

The next Impreza is more than just a new compact sedan
We don't know much about the new Impreza yet. So far, all Subaru has released are a couple of stylized illustrations (including the one above) and a few notes about its styling. It will show off a "concept" version later this month at the Tokyo Motor Show.

But we do know that the Impreza is a big deal for Subaru parent Fuji Heavy Industries. To understand why, we need to know a little bit about Fuji Heavy and Subaru.

Subaru has a big presence in the United States, but by global standards it's a tiny automaker. Toyota (NYSE:TM) (which owns a stake in Fuji Heavy, by the way) out-sold Subaru by more than ten to one last year. It's not even in the top 15 global automakers.

Normally, in the auto business, scale is exceptionally important. Bigger companies have better economies of scale, meaning that they can (theoretically, at least) make more money per sale and invest more money in their future products.

But Subaru has great quality and generates very good profits for its size. That's because Fuji Heavy CEO Yasuyaki Yoshinaga has managed to keep Subaru competitive despite its size disadvantage by being very careful with investments.

With the new Impreza, Yoshinaga and his team have chosen to make a big investment -- because the architecture it will be built on will be shared by all future Subarus.

Why shared architecture will be a huge win for Subaru
No major-league global automaker has ever tried to make all of its vehicles using a single architecture -- at least not in modern times.

"Architecture" refers to a vehicle's engineering underpinnings. First and foremost, it's a set of common measurements that allow different vehicles to share many parts and to be made on a single assembly line. 

Another view of Subaru's Impreza Concept. Image source: Fuji Heavy Industries

Typically, automakers use several different architectures, for different-sized sets of vehicles. They may have a few parts in common, but they're different, inside and out.

Vehicles with a common architecture share lots of parts, but that doesn't mean they're essentially identical. The shared parts are usually where you can't see them. 

For instance, the Subaru Legacy sedan and TriBeCa SUV are built on the same architecture right now. The shared parts save Subaru money: Not only can it order parts in bigger batches, but it only had to design, engineer, and test those parts once -- not once for each vehicle.

The two also share certain key measurements that may not be obvious when you're looking at them, but that allow them to be built using shared tooling. That's important to all automakers, but it's especially important to Subaru. The company has just three assembly plants, two in Japan and one in the United States. 

Subaru already shares a lot of parts and engineering between its models. But by building all of its vehicles on a single platform, it will be able to share even more. It will also  be able to build them all on the same assembly lines, meaning that it can make the most of those three factories and very quickly vary its mix of models in response to market demand.

That kind of flexibility will be a huge win.

A closer look later this month in Tokyo
Officially, the Impreza that Subaru will show off in Tokyo is a "concept", meaning that Subaru hasn't indicated that it's going into production. Sometimes, concepts are flights of fancy, meant to give a general view of an automaker's future direction. 

But sometimes, they're a secret hiding in plain sight: An upcoming new car, wearing a thin show-car disguise. That probably describes the Impreza Concept. It may have fancy show-car wheels, and the designers might make a few small tweaks before it goes into production, but it's a good bet that the Impreza we get late next year will look a lot like the car that Subaru will show off later this month.

It's an important car for Subaru and for Fuji Heavy. But the most important parts might be the ones that aren't easily visible.