Hi there from the United States. Decal softening solution softens the decals and allows them to conform to any irregularities which may be present on the model. I.E. such things as recessed panel lines or rivets. Decal set can then be used to set the decal in place.
I will be honest with you, often I just use the softening solution and find that the decal snuggles down over the plastic detail just fine; without the use of decal setting solution. I enjoy using the Micro Sol softening solution available at many hobby shops.
You will find that solutions come in various strengths as well. Solva Set for instance is very strong stuff and I don't use it often. Only if there is a really stubborn decal which won't conform do I apply it. Hope this helps you out some. All the best, Russell.