Is there any pin for CLOCK in RCM4000 - 50 pin header module?

Question

Which is the clock pin used for slave-master communication where the schematic shows hardware connection between CLK-XTALB1 ?

Answer 1

What is XTALB1?

Comments

It is mentioned in the hardware interfacing schematic diagram(fig.18-1) in chapter-18-Slave Port  in Rabbit 4000 Microprocessor  User's Manual