Which term describes the top of the inside of a pipe?

The term that describes the top of the inside of a pipe is "soffit." In plumbing and drainage systems, the soffit refers to the upper interior surface of a pipe. Understanding this terminology is vital for proper installation and maintenance of drainage systems, as knowing the orientation and dimensions of pipe surfaces is crucial for ensuring effective flow and preventing blockages.

The other terms, while related to pipe components, do not accurately define the top inside area of a pipe. The inner rim typically pertains to the edge of an opening, such as on a fitting or the end of a pipe, rather than a specific interior surface. The term spigot refers to the projecting end of a pipe that fits into the bell of another pipe, commonly seen in various types of joint connections. Lastly, a bell refers to the wider end of a pipe designed to receive the spigot end of another, creating a secure joint but does not describe a specific interior surface of a pipe itself. Thus, "soffit" is the most precise term for what is being asked in this question.