Generally speaking an Adventure Level (AL) is meant to give an indication of the difficulty of the adventure.
PCs within 2 of the AL on either side should be considered prime candidates for that adventure.
For example AL8 means that characters from level 6 to level 10 should be considered within the ideal level range for that adventure.
A variable AL may be provided to indicate the adventure can be scaled up or down to pose a greater or lesser challenge to accommodate the PCs that are available to form the group.
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