The RSTN signal resets the entire chip, MINUS the RTC circuitry and wakeup configuration. This includes resetting the ARM debug engine or JTAG debugger state. SRSTN, on the other hand, is bidirectional and only resets the ARM core and the digital peripherals/registers that are normally cleared by a system reset. The SRSTN does not affect the RTC or POR-reset-only settings. Once either reset occurs, SRSTN is driven low for 6 clock cycles. Additionally, SRSTN is driven low for at least 6 clock cycles due to a watchdog reset, firmware reboot, ARM reset request, ARM lockup, or power-fail event.
Generally speaking, a pushbutton is connected to RSTN and SRSTN is connected to the JTAG interface (SRSTn of the JTAG header, not TRSTn).