The intent is that, when you make the character, you should add up all the normal modifiers for your character for each realm. Stat bonus, level bonus, racial modifiers, bonus for your own realm, etc. Then during play you just add that total to your RR roll and subtract the attacker's level mod (2xlevel). So the target's level is added in advance (since it will be constant until you go up a level), and the attacker's level is subtracted at the time of the attack (since you will be attacked by many different people/things).