The collection is a grouping of products. For example, the H19 collection brings together all the products that make the Winter 19 collection offering consistent style. The collection is not assigned a start and end date, several collections exist for the same date.
A product can be associated with several collections at the same time, this case exists in the context of a renewal. On the other hand, for the same date, its sales and stocks are assigned to only one collection: the active collection.