A typical default is to use 40% training and 60% test. Any dates excluded in GSB left menu will also be excluded.
Custom dates can be used if GSB is in Admin mode.
Periods Mode can be "Percent" or "Dates"
Periods you could use 40% training, and 60 % test.
or you could use 40% training and 40% test and 20% validation.
You could also use dates to specify precise periods in time you want to use. (For advanced users only)