Buy classes refer to buying situations that are distinguished on four characteristics: 
- newness to decision makers
- number of alternatives to be considered
- uncertainty inherent in the buying situation
- the amount of information needed for making a buying decision
There are three buy classes: 
- A new task purchase is a buying situation in which the customer has no previous experience with the purchase of a product within a given category.
- A modified rebuy is a buying situation in which the customer buys goods that have been purchased previously but changes the supplier or some term of a previous order.
- A straight rebuy is a buying situation in which the customer buys the same goods on the same terms from the same supplier.
- American Marketing Association, AMA Dictionary.
- Common Language in Marketing Project, 2021.