The phrase "soft surface" refers to a material or area that is cushioned, gentle, and not hard or rough. It is comfortable to touch or walk on, and usually lacks firmness or rigidity. Full definition
One of the most important things that you can do for your pet is to exercise them regularly by taking them for short but frequent daily walks on soft surfaces like grass. (tampabayvets.net)
For example, a cat that finds it pleasing to eliminate on soft surfaces like clothing or carpets would be unlikely to use tile floors. (www2.vet.cornell.edu)
If there are potty pads all over the house then it is easy to confuse potty pads with other soft surfaces like carpet. (dogtopia.com)