A multiplexed input tap is an input peripheral signal, which is automatically enabled when the peripheral is enabled and it will use the signal on the pin. i.e., That particular port pin can be enabled in any MUX mode.
For example in ADSP-21569, if PORTA_07 is configured for UART0_RX, when Timer 0 is enabled for using TM0_ACI0, the signal on PORTA_07 (UART0_RX) will be internally routed to TM0_ACI0. This can be used to measure the baud rate of UART in Timer when UART is active.