Every winter tire has two kinds of wear indicator bars on it.
Why? It’s simple, really. It is only the studded winter tires that the law prohibits to use in the summertime, and only in the summer months. Otherwise, there is no prohibition.
About this, as well as about why you must not use winter tires in the summertime.
So, why does the winter tire has a 4mm «winter» wear bar and a 1.6mm «summer» wear bar?
The manufacturer is legally bound to equip the tire with a wear indicator bar, below which the tire is not to be used under any circumstances. Essentially, this allows the motorist to «use up» the worn studless winter tire in the summertime. It will perform better than a new spiked winter tire, but still worse than a dedicated summer tire.
In the figure above, the green circle marks the summer wear indicator bars, and the blue circle marks the only winter wear bar. As a rule, summer wear bars are placed in each of the circumferential grooves, while the winter wear indicator is only placed in a couple of spots, like in this example.