The rules and etiquette are:
If the seat is marked as reserved, either with a paper or electronically, for your specific booking, you can ask someone to move out of your reserved seat.
If the seat is marked as reserved by someone else but the person doesn't show up, you can sit there until their reserved station (and beyond if it doesn't interfere with someone else's reservation). It can be good to keep an eye out for these seats as they often get overlooked/missed, especially on crowded trains that have a lot of stops as many people just see "Reserved" and don't check the stops it's reserved for.
If the reservations are down or not in operation, which is usually but not always announced by the manager, and otherwise denoted by lack of reserved signs, no one has a reservation so even if you've reserved that seat you can't ask someone to move out of it.
If the reservations are working as normal but the train is empty and it's not a busy route and you ask someone to move out of your reserved seat, they have the right to make grumpy noises about it to acknowledge your pettiness (assuming it's not for health reasons) but they should still move anyway.
If you've reserved a table seat and two of the three other people don't show up, etiquette is to move so you're sitting diagonally to the other person.
If you're sitting in (proper) first class with strangers next to or opposite you, you should move out of your reserved seat to a different set of seats to give people more space. Tip here is that there are usually at least 2 first class carriages and they only reserve seats in one, so the other typically has far fewer people and you'll usually get a table to yourself. So always check the other carriage.