N: you're a non-EU resident and you purchase items in the EU for personal use in your non-EU homecountry
E: you export the goods to your non-EU homecountry before the end of the third month, after the month of purchase
M: you spend a minimum amount per store, per day
S: you've received a stamp from customs on your receipt or other export documents, either from EU-customs or from customs in your non-EU homecountry
Other eligible groups include:
Please also check the requirements for purchase receipts.