Every time when you generate invocation tag the "n" value will be changed even for the same zone. It represents a random generated string for cookies' name.
The cookies keep information about the banner that is shown so each invocation tag on a single page should have a unique value for the "n" variable.
This way will ensure that click on the banner will redirect a user to the valid destinatoin. Invocation tag should have the same "n" value in itself, e.g. image and link tag need to use the same value for "n" variable.
Value of the "n" variable doesn't have strict rules so you are allowed to generate it by yourself. It only needs to have 8 alphanumeric characters and of course pay attention not to use any special character due to possible problems with encoding.