Restricted WoE was introduced to bring a more even level of competitiveness for lower level guilds in War of Emperium. Certain WoE castles were limited by certain restrictions in gear and items.
Specialty potions created for Restricted WoE are both 90 minute rental items with unlimited use. You can obtain these potions from the Potion Master NPCs standing outside the castle warps. The Potion Master NPC will start handing out potions 30 minutes prior to WoE and will stop giving them out once WoE ends.