MPC5534: p_tbclk input

At page 2-28 of "e200z3 PowerPC Core Reference Manual" (e200z3RM Rev. 
1, 03/2006) you can read this:
"If HID0[TBEN]=1 and HID0[SEL_TBCLK]=1, the time base and decrementer 
are based on the p_tbclk input".
What is the "p_tbclk input" on a MPC5534 microprocessor?
TIA