There's a bit more description in Help/Language/Modules (if you haven't already seen that).
If it was FA stating that BRL.basic is all your program needs, then it probably is. I've found it to be pretty comprehensive.
If you need to add on other modules, use the Import command (FA is good about including those as needed). Here's an example of a header produced by FA when I was only using Print and Input commands:
Framework BRL.StandardIO
And here's an example of one using graphics, GUI, network and other stuff:
Framework BRL.D3D7Max2D
Import BRL.LinkedList
Import BRL.StandardIO
Import BRL.GNet
Import BRL.EventQueue
Import BRL.MaxGUI
Import BRL.Win32MaxGUI
Import BRL.BMPLoader
SetGraphicsDriver D3D7Max2DDriver()
But, again, all the Framework/Import stuff is optional. It's not like C where you must explicitly include modules in order to use them. You can just ignore Framework/Import while learning BlitzMax and you'll have access to everything by default.