It is pretty much situation by situation, but these are the key rules that apply in these situations:
12.8. Every player is entitled to occupy any position on the field not occupied by any opposing player, provided that they do not initiate contact in taking such a position.
12.9. All players must attempt to avoid contact with other players, and there is no situation where a player may justify initiating contact. “Making a play for the disc” is not a valid excuse for initiating contact with other players.
17.1.1. Reckless disregard for the safety of fellow players is considered dangerous play and is to be treated as a foul, regardless of whether or when contact occurs. This rule is not superseded by any other rule.
17.8.1. A Blocking Foul occurs when a player takes a position that a moving opponent will be unable to avoid and contact results.
The Interpretations for these sections provide additional details about these rules, but the basic idea is this:
– If you are in a position that an opponent could reasonably avoid, and you strongly feel like someone is about to crash into you, you can call “Dangerous Play” and then get out of the way.
– It is not reasonable to run without looking where you are going and then complain that you ran into someone, unless that person moved in front of you at the last moment, in a way that still would have initiated contact even if you had been looking.
Note: These plays generally result in contested calls because the people involved in these collisions have a very poor perspective on what happened. Teammates who did have a good perspective should provide this perspective, even if it means telling your teammate that they have made the wrong call.