What are pre-requisite Quests?
There might be special conditions to join certain quests. Some quests may have pre-requisite quests. In such cases, you need to have at least joined the pre-requisite quest, before you can join the next quest.
What are the repeated Quests?
Some quests may be repeated quests of previous quests. In such cases, if you ever submitted AND got a reward for the previous quest, you will be unable to join this new repeated quest, to give other players a fair chance.