A data group is the link between your master sheet and a secondary sheet. It’s what lets one record reach out and collect its own related rows.
Your master sheet has one row per record. A secondary sheet has many rows that belong to those records. On their own, DataMergeStudio has no way to know which secondary rows go with which record. A data group supplies that connection: it matches a shared value on both sheets – a product’s SKU, a customer number, an order ID – so each record automatically gathers the secondary rows that share its value.
This is the capability InDesign’s built-in data merge simply doesn’t have. With a data group in place, one product document can render its entire spec table, and one invoice can list all of its line items – each record pulling exactly its own related data, no more, no less.
A data group isn’t created while you import – it’s configured afterward, in the datasource settings, once both sheets are in your workbook. You choose the master sheet, the secondary sheet, and the column they share.
See the matching in detail: How Data Groups Link Master and Secondary Sheets.