Targetable
A unit is targetable by another unit if it is in rangeRangeRange (or distance) from one hex to another is measured by counting the number of hexes between those hexes, including the destination hex, regardless of whether those hexes are occupied. and sightSightA unit has sight to another unit (or hex) if both units are in hexes on the same tile or an adjacent tile. of the unit that would target it and does not have an effect that prevents it from being targeted. An adventurer's range is limited by their current battle formBattle FormsBattle form icons appear on skill lines, abilities, and weapon items. During an engage action, the adventurer's battle form limits the dice that can be rolled together and sets the adventurer's range.. An enemy's range is limited by its range stat. Some abilities can apply effects to targetable units outside of an engage action or allow a unit to target allies. Some abilities can also deal damage to units without targeting those units.
The target has to be in range when selected as a target and when the die is resolved.
SourceDeclaring a Target
You only declare a target if the game structure tells you to. i.e. as part of an engageEngageAdventurers may engage multiple times per turn but are limited by the dice they can roll and fatigue they can gain. You may perform a move action before or after you perform an engage action, but not both., or if something else in the game tells you to select a target. You can't just randomly select yourself a target in order to use something that triggers based off of selecting a target.
Do abilities that affects "targetable units" become your target?
If an ability affects "targetable units" those units do not become your target.
Source