All of the above is correct. It just depends on what you are filling. If you have a defect that is well supported like a ding or gouge, then spackle will work well. And, it is easy to sand smooth after it dries. If there is a possibility of movement like with two adjoining pieces that are nailed in place, then caulk is usually best. Spackle will crack and fall out under movement. However, as mentioned above, caulk won't easily fill deep or wide defects.