Aiming a Spell wrote:
Target or Targets
Some spells have a target or targets. You cast these spells on creatures or objects, as defined by the spell itself. You must be able to see or touch the target, and you must specifically choose that target. You do not have to select your target until you finish casting the spell.
Some spells restrict you to willing targets only. Declaring yourself as a willing target is something that can be done at any time (even if you're flat-footed or it isn't your turn). Unconscious creatures are automatically considered willing, but a character who is conscious but immobile or helpless (such as one who is bound, cowering, grappling, paralyzed, pinned, or stunned) is not automatically willing.
Some spells allow you to redirect the effect to new targets or areas after you cast the spell. Redirecting a spell is a move action that does not provoke attacks of opportunity.
Creatures
A spell with this kind of area affects creatures directly (like a targeted spell), but it affects all creatures in an area of some kind rather than individual creatures you select. The area might be a spherical burst, a cone-shaped burst, or some other shape.
This difference is determined by the "Targets" under a spell. Sleep chooses an area instead. So with Sleep you cannot exclude targets unless you have something like Selective Spell as a metamagic applied to the spell. With Mass Bull's Strength or Mass Cure Light Wounds you can exclude creatures. In this sense you could exclude allied undead or those that already have the buff. Or someone in your party you just don't like lol
