A zero value disables the extra person function.
There are two options to set an extra person price by specifying either a positive or negative value:
i.e. One person booking a daily price set for two people will receive one extra person discount. Three people cannot book this daily price.
Note: There are limitations for channel management which are different for each channel. Most channels cannot accept the extra person price. Please see the channel help page for information if/how extra person prices can send.