The information you can find in S12CPUV2 manual:
See chapter 7.5 Interrupts
In stop mode for example, low signal on IRQ pin triggers IRQ_ISR. Prior to stop mode IRQ must be enabled by setting IRQCR register and bit IRQEN.
Also, low level on XIRQ pin wake up the MCU.
However, if XIRQ interrupt is disabled then the code continues on the next instruction after STOP.
If the XIRQ interrupt is enabled then the XIRQ_ISR is triggered.
Hope the information helps.
Both can be disable/enable, but the XIRQ does not have dedicated register like IRQCR.
In case of XIRQ, after minimum system initialization, software can clear the Xbit in CCR register using an instruction such as ANDCC #$BF.
This way the /XIRQ interrupts are enabled.
After return-from-interrupt (RTI) instruction at the end of the interrupt service routine (ISR), the X bit is restored to initial value (set to logic once) and XIRQ is disabled.