Well if it's during an interaction everything is significant. A low (not between legs but just held low) wagging tail can be a calming signal of a fearful dog trying to appease you (like if he is fearing a punishment he uses it as a sign he doesn't want trouble). when you were petting him if it was on top of the head they often will pull their ears back since patting on the head is a somewhat rude unpleasant thing for them. Sometimes he may just be holding his ears like that for reasons you might never know, I know my cats hold their ears in the "I'm going to kill you now" position when the TV is at an unacceptable volume for their very good hearing. I'm no expert in body language but from my experience I usually see dogs holding a tail parallel to the ground when interacting with a dog they are unsure about, it's almost like their tail is trying to pull them away from the other dog lol. If the hackles are also raised it could mean aggression, showing status etc. You really have to rely on more than one part of the body to get the whole message though. A dog growling can have a wagging tail and his ears back so it's not pure aggression or pure fear it's a little fear, a little unsureness/nervousness and some defensiveness.
watching a dog's body is very important though and a skill EVERY dog owner should learn to do with their own dogs. being able to tell what your dog is telling you while the signals are subtle, a small change in the way he's breathing, holding his ears, focusing or not focusing his eyes, tightness of jaws and lips, position of tail, will allow him to rely on you to understand him and leads to him feeling respected by you. Dogs think people can understand what the are saying but simply are ignoring there signals when in fact we have NO idea that they are even telling us anything. To some extent you do have to learn your own dog's language since every dog will "speak" in a slightly unique way whether it be in intensity or style.
Sit down with your dog and pet him, watch him for any change in anything. When you think he may be subtly asking you to do something whether it be stop touching there, scratch harder, scratch over here, pat me here try listening to it and see how he responds if you stop and he seems fine with it chances are you guessed right. If you stop and he looks puzzled chances are you guessed wrong. when learning your dog's language there will be a lot of guessing wrong but at least you know what one signal does not mean so you won't respond to it like that again.
Sorry for the longer than needed post, I'm reading a dog behavior book so I tend to summarize my reading in a way to make sure I got it all down in my head and didn't miss anything that needs re-reading lol.
