Final answer:
Threads in drainage fittings are tapped to securely hold components and accommodate attachments like devices that increase flow rate or prevent water lock.
Step-by-step explanation:
The question pertains to plumbing, more specifically, to the design of drainage fittings. In such systems, threads must be tapped to allow a specific feature or function.
Although the question does not include sufficient context or information to provide a specific answer as to what threads in drainage fittings should allow for, these threads are generally tapped to ensure they can securely hold components together and to make sure they can accommodate attachments like drainage devices or valves. These devices could be designed to increase flow rate or ensure proper air flow in the drainage system to prevent vacuum formation that could hinder efficient drainage.
Devices like those shown in Figure 12.28 are likely to be air admittance valves or similar fittings that allow air into the drain system to balance the pressure, preventing water lock and enhancing flow efficiency.