Settings - SYNC Tab
The SYNC tab is where you will link your external Steam and Paizo accounts with your Fantasy Grounds account. This will allow your Fantasy Grounds account to see what you have already purchased via Steam or Paizo, and either download the purchased DLC (Steam) or offer the appropriate discount for purchasing the .PDF of a corresponding Fantasy Grounds module (Paizo).
For a complete step-by-step tutorial walking you through the process of linking your Steam or Paizo accounts, please see the following help article:
Link: Linking with External Services and Applications