NI MAX Sample Clock Settings

Hello

I am using the bridge module ni 9237 along with the NI cDAQ 9185. 
While creating a task on NI MAX, i want to configure the sample clock settings.

I have the option to choose between internal or external type of clocks.
Does internal mean module based i.e. (ni 9237) ?
Or does it mean the DAQ clock.

what does the external onboard clock mean?

Also can't I choose the cDAQ 9185 timebase 10HHz as source of clock?

Thanks for your help.