Data Sources are how your API gets populated with data.
Currently, there are 3 different supported types of data sources.
For Google Sheets and Airtable data sources, the destination table they sync to must be unique. For example, you are not allowed to sync both Google Sheet data and Airtable data to the exact same table.
You're also not allowed to sync multiple Google Sheets, or multiple Airtables (by design), to the exact same table.
These rules are put in place to prevent complicated data integrity situations from taking place: it's challenging enough to successfully synchronize data between these platforms and an API, introducing another data vector where data can get intermixed could cause some data corruption. In an effort to avoid those sorts of situations, we've put some restrictions in place.
These restrictions do not apply to External API, however, as it has been designed to complement the data already present within your Google Sheets or Airtable.
For more information on these various types of Data Sources, check out their individual pages.