When each is allowed, the grip trade-offs, and why you must never mix them.
There are two types of approved winter tyres: studded (metal studs for ice grip) and friction (studless, softer rubber for cold weather).
Under TSFS 2009:19 8 § studded tyres may not be used from 16 April to 30 September, unless winter road conditions prevail or can be expected and that justifies them. In practice that leaves them permitted 1 October to 15 April, plus any day outside the window when winter genuinely turns up. Notice how the law is written. It does not grant a season. It prohibits one. That is why the exception exists at all.
Studded: SHORTER braking distance on ice and packed snow, LONGER braking on dry and wet asphalt. Friction: better balance year-round but slightly worse on pure ice. Choose based on where you drive most.
Under TSFS 2009:19 10 § a vehicle may not run studded and non-studded tyres at the same time. All four must be the same type. And under 11 §: if the car is on studs and winter road conditions prevail, the trailer's tyres must be studded too. Winter tyres on the trailer are not enough. They have to match. Most drivers know the trailer needs winter tyres. Far fewer know it has to match the car.