3v3 Digital vs Analog

Hi,
regarding the DGND and AGND i already understood that i have to connect them both together.
But i do not understand when to use 3v3 Digital (pin 38) or 3v3 Analog (pin 21) in a cirquit (for instance on connecting LED, Poti, or more sophisticated things like bus connections). Is there a simple rule that helps me to understand when to use which ?
Georg