what's clock reference frequency when calling es_out_Control

[xzou999]

When calling es_out_Control(p_demux->out,ES_OUT_RESET_PCR), it may call the function input_ClockSetPCR( p_sys->p_input, &p_pgrm->clock, (i_pcr + 11 ) * 9 / 100) to set the Program Clock Reference. I have read the inside of input_ClockSetPCR,but still don't understand what value was the reference clock frequency.
Can anybody help