Targetable
A unit is targetable by another unit if it is in The fallback content to display on prerendering and The fallback content to display on prerendering 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 The fallback content to display on prerendering. 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 The fallback content to display on prerendering, 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