Areas With Targets
Some effects have areas that target creatures or objects within the specified area. Unlike for targeted effects (see Target on page 272), you do not get to select which creatures are affected; the effect affects all creatures or objects of some kind in the specified area.
If an effect restricts which targets are affected (for example, it affects only living creatures), then creatures in the effect’s area that are not of the appropriate type do not count against the number of creatures affected.
Source
Core Rulebook pg. 268