Water Soluble Topping is usually a very light weight see through Film Type of water soluble such as light weight Solvy.
It is usually used to hold done the nap of a fabric so the the needle doesn't get caught on the nap and also to help the stitches to stay on top of the nap of the fabric while you stitch.
There are 2 types water soluble backing stabilizers.
Both are used for embroidering Free Standing lace or for doing cut work embroidery.
One is a heavy weight Film type of water soluble such as Badgemaster. It is much heavier in weight and not as see through as the light weight water soluble topping.
The other is a Fibrous Type of water soluble that looks like a very light weight tear Away stabilizer but is water soluble. There are many brand names for the Fibrous type of water soluble such as Vilene water soluble, Aqua magic, Wash Away, Wet N Gone (Floriani), H2O Gone, Avalon Plus.
Both types of the water soluble backing stabilizers will stand up to the large stitch counts in Free standing Lace designs and the many layers of stitches in the designs.
You could use either for a topping stabilizer but they are more expensive than the light weight water soluble topping.
EastWitch2
You are correct WSS for topping is used so that the design doesn't sink into fabric.
WSS backing is as Marjialexa has said.
Hugs Yvonne
thanks, read hers right after yours and now it totally makes sense - thanks.
The WS backing is heavier, looks more like cloth, that's what you use when making free standing lace, because it needs to be heavy to support all those stitches without tearing. The WS topping is the one that looks like plastic wrap, that's what you put on towels, velvet, etc. to keep the nap in and give you clean edges to your embroidery. Of course you still use cutaway or tearaway on the back. I guess you could use the topping type to make lace, if you used enough layers, but just the construction of it makes it more fragile, and the repeated needle holes make it tend to tear away when you don't want it to. The heavier WSS cloth type is made to stand up to repeated needle punches. Hope this helps. Hugs, Marji
The water soluble topping is thinner than the water soluble backing and is used on top of towels or velvet or similar fabrics to keep the design from becoming lost in the thicker fabric. It also keeps the towel loops from catching in your embroidery foot. Hope this helps.
The way I see it Vilene is vilene. It differs by manufacturers - formula, weight, elasticity are the most important factors, so if I need more - I stop my machine and put more on! top or bottom! Live Laugh Download Stitch!
Probably in the application. For example, if you are making FSL, then it would be used as a backing. For monograms on towels, etc. then it would be a topping. That is my take on it anyway. Please someone correct me if I'm wrong.
You can also use it as stabilizer. An example would be for lace designs.