My father-in-law used to take a big fork and wrap white cotton cloth around it. He tied it with string and called it his "sop fork." He dipped it into a sauce he made with vinegar, crushed red pepper, garlic and black pepper. The entire time he was cooking chickens, he would "sop" the chicken. Is this what you mean? It's the only way I'm familiar with the term. I think he called any sauce his sopping sauce.
Sops, also called mops, are a great way to keep moisture in the meats your smoking. As for flavor, it's up to you. When I use them, which is seldom, I go for something that compliments my meat. If the the meat rubbed on the sweeter side, like pork, I go for a touch of heat.