Wheel Well Liners are molded out of  a tough polyethylene resin with a smooth radius that seals off the opening between the inside of your trailer and the road. 

 

The Type A wheel wells are molded with a flange that goes up and seals against  the sidewall of the trailer. That flange can be trimmed off as an option.

The Type B wheel well molded without that flange and can't be added