The content of the IRC was developed and has been maintained to provide builders and inspectors with a single code book that covers all aspects of typical home construction. If the subject matter is covered in the IRC, then it is intended that the IRC will provide all of the needed coverage for that subject. Therefore, it is not intended that the IRC be "customized" with additional requirements plucked from the other codes.
IRC users expect all of the applicable requirements to be in the IRC and do not expect to have to determine what additional unknown requirements might apply from some other code. If this was not the case, IRC users would never be sure that all of the applicable requirements were in front of them. The IRC stands alone except when it specifically directs the user to another code. The fact that the IRC does not contain some particular requirement from another code does not mean that the IRC is somehow deficient in its coverage of the broader subject. For example, the IRC covers the design of drainage system piping, as does the International Plumbing Code (IPC). If the IPC contains an additional requirement that is not also in the IRC, that requirement is not to be applied to an installation regulated by the IRC.
In this case, the IRC contains all of the coverage for drainage piping that was intended. In other words, it is not intended that the IRC provisions be "mixed and matched" with provisions of other codes and it is not intended that code users "shop" in or "cherry pick" from other codes when applying the IRC. It is likely, that where the IRC does not contain a particular requirement that is found in another code, such requirement was intentionally left out of the IRC or it relates to atypical construction. See Sections R301.1.1. N1101.2, M1301.1, G2401.1, P2601.1 and E3401.2.