Minecraft wiki entry for Block_entity_format, associated text: : The block entity's root tag.
 primary: The primary effect selected, see Potion effects for IDs. Set to 0 when no effect is selected.
 secondary: The secondary effect selected, see Potion effects for IDs. Set to 0 when no effect is selected. When set without a primary effect, does nothing. When set to the same as the primary, the effect is given at level 2 (the normally available behavior for 5 effects). When set to a different value than the primary (normally only Regeneration), gives the effect at level 1.[needs testing]