Expert magic numbers are ids given to a particular expert advisor.
Care should be taken to ensure the ids are unique.
This is because the magic number is used by the MetaTrader back end to know which expert advisor to send information to. For example, when a trade notification is received, MetaTrader will have stamped the order with the expert advisor’s magic number, and it therefore knows which expert advisor to send the trade notification to.
It can therefore also be used to find out which expert advisor sent a particular order.