On various marketplaces, you can offer product families in order to offer different products that belong together. For example, think of a pair of shoes in different sizes. When you have created a product family the marketplace will show these products in a collection. 
How you create these families can differ. Some marketplaces (such as bol.com and fonQ) offer the possibility to create product families via the mapping groups.
When you create product mappings, look for a column that asks for a 'parent ID'. In this column, you always place an identifier, which characterizes a family. So, for example, if you have SHOE-A, SHOE-B and SHOE-C, and want to add them in a family, all 3 of these products must have the same parent ID.
Don't have a parent ID? You can solve this by adding a new attribute with a parent ID in your source data (e.g. webshop), or you can use custom attributes to provide your products with a parent ID.
Can't find a parent ID in the mapping group? Then please contact the marketplace and ask them if they can inform you on how to create a family.