Is the standard cylinder diameter sc 125*25 of Yadek aluminum alloy cylinder double-acting or single-acting?

The sample says it's dual-use. You can go to official website to see it yourself, as shown below.

The single-acting type will be marked with a return spring, while the double-acting type will not.

SC is a standard double-acting cylinder.

SCD is a double-headed piston rod of a double-acting cylinder.

SCJ is a double-acting cylinder with adjustable stroke.

There is an S at the end, and the piston has a magnetic ring.

So your cylinder is a standard double-acting cylinder with a cylinder diameter of 125 and a stroke of 25, without a magnetic ring.