For example, if someone is cutting in line in front of me at a store and I'm sure it's not to go somewhere outside of the line, I might say "excuse me" out loud to be like get in line, and if they still ignore I might repeat myself and be like "Excuse me, you're budding in line". And if they're purposefully ignoring, I'll just walk in front of them while staying "smiley". I may come off as a dick, but it's like the other person caused that situation, so at the point to me at least it's just an enforcing of boundaries.

But what about a situation where another person is being disrespectful but they didn't cross a line for a boundary to be enforced? Like someone calling dibs on a chair that you usually sit in and since they're not going to budge, you need to sit at the edge of the table. Causing a fuss seems to just come off as being butthurt and overly emotional (even if the person was being a dick in the situation or knows what they're doing is lacking in etiquette), so what I tend to do is just sit at the edge and act like nothing happened.

So how can you tell the difference between when to back down and when to enforce?