A Replacement rule is a rule that states that a cetrain off-game action is used to replace an action in the fiction. Replacement rules are usually made to allow for actions in the fiction that are either impossible to conduct or that most people consider unacceptable to perform. Replacement rules are often use to shield the players from player fatigue originating from violence, sex, hunger, thirst, lack of sleep, lack of shelter or lack of sanitation.
Formal vs informal replacement rules
All larps have implicit replacement rules in force. These are prompted by common sense and by the culture of the community where the larp is played. For example, in most communities it would be unacceptable for players to actually be violent to each other even though their characters are violent. At larps where one doesn't expect violence to occur, the players will still understand that they should just pretend to be violent instead of actually hurting each other, even though this has not been specified as a formal replacement rule.
Formal replacement rules give a distinct action to use instead of another. For the organizers can instruct the players that if their characters are going to have sex, the players will massage each other to replace this action. Or it could be that if a character is conjuring a fireball by magic, the player throws a ball of orange cloth to replace this action.
There are different opinions on whether actions such as light violence, psychological violence, physically dangerous performances or light intimacy (e.g. kissing) should be replaced. While it is not common, a few larps have even abstained from replacing sex. Which actions that needs to be replaced is highly dependent on the community where the larp is being run. Still, there will probably be consensus among all larpers that at least the act of killing someone must be replaced.
So-called hardcore larping is an approach to larp design that seeks to minimize the use of replacement rules. Proponents of the hardcore approach often argue that it is easier to have an intense immersive experience. The opposing view (labeled as pretense in The Mixing Desk of Larp is that lack of replacements may make the players less able to roleplay properly, for example acting out of character to get food if the character is very hungry or steering the character towards intimate situations because the player wants them. In many situations lack of replacements can lead to player fatigue which makes them less able to roleplay their character.
Degree of shielding
Different replacement rules have different impacts on how much the impact on players is reduced. For example, if sex is replaced with another intimate action such as Ars Amandi the degree of shielding is less than if it's replace with a rather un-intimate action, e.g. nodding to each other.
The players can also be given the choice to opt for replacements themselves. In the larp Kapo playing on lack of sleep and lack of food was part of the design. However, players who felt they were too prone to player fatigue could go off-game and eat or sleep, effectively giving them the option to replace hunger with pretended hunger or exchaustion by pretended exhaustion.
Hardcore approaches where replacements are not used can also be used as a design tool to influence the threshold for when to use an action. In the larp Europa no replacement were used for fighting. This was actively stated from the organizers in order to overrule the default informal replacement rule in most communities of replacing fighting with play-fighting if nothing else is stated.
Even when all players have given informed consent to this and there are proper opt-out mechanics in place, this lack of replacement has implications for player safety and possibly player fatigue. However, it also severely increased the player thershold for engaging in a fight, thus eliminating or heavily reducing fighting as an aspect to play on.
On the opposite end, enabling replacements for actions can easily encourage these actions to take up more space in a larp. For example, if you spend a lot of time during a pre-larp workshop exercising a replacement rule for sex, it can give a signal that sex is an important part of the larp in question. Furthermore, if the replacement rule lowers the bar for conducting the action, it can encourage it to be used more often than what would be plausible in the larp in question.