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From the 2015 UPC Illustrated Training Manual

310.0 Prohibited Fittings and Practices.
310.1 Fittings. No double hub fitting, single or double tee branch, single or double tapped tee branch, side inlet quarter bend, running thread, band, or saddle shall be used as a drainage fitting, except that a double hub sanitary tapped tee shall be permitted to be used on a vertical line as a fixture connection.

Figure 310.1 shows examples of the prohibited fittings and practices addressed in this section. These fittings are prohibited because either they are not true drainage fittings with contoured edges to allow proper flow into and down the pipe or they contain obstructions that may cause blockages.

The "tee branch" fittings referenced here, such as the cast iron twin tapped tee, are not "sanitary tees." Essentially, a flat surface was cast onto the walls of these branch or fixture fittings, then this flat surface was drilled and tapped to accommodate threaded pipe or trap adapters. Because the branch opening on these fittings did not extend beyond the walls of the fitting (as is the case with a sanitary branch), any pipe or fitting that was screwed into these sidewall taps extended into the barrel of the fittings, thus obstructing both air and waste flow. These fittings were prohibited many years ago.

310.2 Drainage and Vent Piping. No drainage or vent piping shall be drilled and tapped for the purpose of making connections thereto, and no cast-iron soil pipe shall be threaded.

The thickness or material in the wall of any type of drainage or vent piping is not sufficient enough to form a tapped thread. Approved adapter fittings are required whenever drainage pipe is joined with dissimilar materials.

310.3 Waste Connection. No waste connection shall be made to a closet bend or stub of a water closet or similar fixture.

A closet bend is an approved fitting that may include an integral or attached stub receiving vertically discharged waste from a water closet and changing its direction of flow, usually to horizontal. The closet bend is a trap arm up to the point of fixture vent connection. A waste connection shall not be made to a trap arm before its vent. The waste flow that would occur in the trap arm from this installation could cause venting problems. Figure 310.3 shows an illustration of a prohibited closet bend with side inlet.

The 2015 Uniform Plumbing Code Illustrated Training Manual eBook is available for purchase here.