What is the criteria to blend the data from multiple data sources?

The requirement is a shared dimension — a field common to both sources that Tableau can match on, called the linking field.

Tableau proposes links automatically where field names and types match. You can edit them under Data → Edit Blend Relationships, mapping fields whose names differ, and activate or deactivate a link by clicking the chain icon in the Data pane.

The values themselves have to correspond. If one source stores `USA` and the other `United States`, the blend produces nulls until you alias one side or create a calculation that reconciles them.

Beyond that: the first source used in the sheet becomes primary, secondary measures are always aggregated, and you cannot get row-level detail from the secondary.