The Multiplex interrupt (Interrupt 2Fh) provides the Microsoft-approved way for a program to verify the presence of an installed TSR and to exchange information with it. DOS version 2.x uses Interrupt 2Fh only as an interface for the resident print spooler utility PRINT.COM. Later DOS versions standardize calling conventions so that multiple TSRs can share the interrupt.
A TSR chains to the Multiplex interrupt by setting up a handler. The TSR's installation code records the Interrupt 2Fh vector and then replaces it with the address of the new multiplex handler.