what is the real purpose of this setting? what will happen if i select this to sm mode? thanks
The history behind this setting is to allow customers to run an AP as an SM in an emergency mode, but more useful, is that historically, the AP had to be in SM mode in order to run the Spectrum Analyzer feature.
As of 13.1.3, the ability for the AP to run the Spectrum Analyzer feature natively has been available for the PMP family (100, 430, 450). The setting is still there since some customers would have had scripts in place that relied on this operation in order to run Spectrum Analyzer, and we didn't want to break those.
On a PMP450, as of Release 13.2, the AP can't run operationally as an SM, so the usefulness of this setting is purely for legacy reasons. For PMP430/PMP100, you can still run the AP as an SM operationally, so the emergency SM replacement reason is still valid for those platforms.
Hope this helps!
Thank you so much… By the way, do you know how to get the gateway address of a router on a tower by using wireshark? Thank you 
In the past, I have found it helpful to set an entire AP cluster on a tower to act as SMs and using the AP Evals from them to identify sources of noise. The trick is to enable all frequencies to allow the AP Eval to function, and to change the colour code so you don't connect to another tower and create a loop.
It will only show you other Cambium products in the same line, and is less useful with the increased variety of products in the area these days, but you may still get some use from it.