Drain caps cover the open end of a drain pipe. Some have a threaded insert that plumbers remove to clean out the line when debris or roots plug it. However, the caps are also useful when plans include the extension of the line.
A common complaint with sewer drain caps that have a cleanout plug is a stuck plug. Heat is sometimes a useful tool for removing the plug. With cast-iron plumbing, a propane torch supplies the necessary heat. When working with plastic pipe, the torch can melt the pipe, so homeowners should stick with a hair dryer to supply the heat.