Hi @fengba,
Please take a look into the OSIF module, function void OSIF_TimeDelay(const uint32_t delay).
The reload value of the SysTick is the core frequency divided by 1000, that means that the Systick interrupt is called every 1ms and increment the s_osif_tick_cnt counter.
The counter is then read and compered by the OSIF_TimeDelay() function.
If the core frequency was divided by 10000, the counter would increment every 100us (OSIF_TimeDelay(1) = 100us).
The SysTick description can be found in ARM documentation: Cortex™-M4 Devices Generic User Guide, Section 4.4 System timer, SysTick.
Best regards,
Julián