Wikia

Quest items are generally items used, made, or obtained only during quests. These include:

Items that are used only in the quest, but not anywhere else, like the Red cog from the Clock Tower
Rewards from quests, like Iban's staff from the Underground Pass
Unofficial rewards, like the Blurite sword from The Knight's Sword
Items that may be kept after the quest is complete, but only have a use during the quest, like the Diary of Herbi Flax from the Shades of Mort'ton
Items that may be kept and used after the quest is complete, like the Dramen staff from the Lost City

Quest items do not include:

Common items, even though they are required for quests, which can be obtained and used without starting or completing any quests, like Buckets and Crushed gems
Items not used during quests, like the Security book
Food, like Lobster
Items that are unlocked by quests, but must be purchased, like Dragon longswords and Rune platebodies, or can only be obtained after the quest, but are not a direct reward for the quest, such as unique Temple Trekking items.

Quest items