The Debuff Warning tab controls how alerts are sent on debuffs.
The dropdowns have the same options for all settings, alerts can be when within spell range, within 100 yards (visible range) or always.
Options
Change health bar colour - Change the bar colour to the setting defined.
Change aggro bar colour - Change and the aggro bar colour to the setting defined, aggro bars will show even if the player has no aggro.
Display warning on debuff - Display a text warning.
Play sound on debuff - Play the sound set on the slider below the option.
Raid filter - Filter out monitoring certain groups by deselecting the group numbers, this option applies to all frames when in raid.