Polo Rules

When the rules say "a penalty will be assessed", does this mean it's at the ref's discretion?

E.g. the high-sticking rule - playing a high ball with no-one around you might be judged OK, but if there are other players nearby it becomes dangerous and a penalty will be called?

Yes, a ref should decide the severity of a penalty, based on how the game was affected, safety, etc...

Personally I'd turn a blind eye to a high stick that was far away from any other player, but yes, if it was a dangerous situation, then I would.