Single Sign-On allows users to log in to a cPanel & WHM or Plesk hosting account directly from a link in WHMCS without reauthenticating.
Application Links use WHMCS Single Sign-On to allow users to seamlessly access WHMCS from the cPanel or WP Squared control panels.
Single Sign-on allows trusted applications and third parties to log users in to WHMCS automatically, without reauthentication.