There are many different things a clue may ask you to do to complete it. The Treasure Trails guide is the best source for information on clues.
If you have a clue you are unable to complete you can talk to Zaida in the Treasure Trail hub south of the Varrock Grand Exchange and she will replace it with a lower level clue.
A clue is completed when the reward casket is granted not when the reward casket is opened. In the same way Treasure Trail points are awarded when the reward casket is granted not when opened. Reward caskets stack in a players inventory or bank based on level and can be opened at any point in any quantity desired. Upon completing a clue you will receive a message regarding the points earned, total points, and total clue completed for that tier of clue.
The Reward caskets provide the rewards for completing clues. There are five levels of reward caskets that correspond with the five levels of clues: (easy), (medium), (hard), (elite), or (master).
A set of instructions to be followed.
Or, for a coordinates clue:
Perhaps someone at the observatory can teach me to navigate?
Or, for a map clue:
A part of the world map, but where?
Or, for a scan clue:
It points to great treasure!
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