Source is where data orginates from. It can be an API, a S3 bucket, Snowflake Table, etc.
Connecting to a source usually requires credentials such as authentication token, user, password. Upon selecting a source, a customer needs to fill in the required credentials and test the connection.
Configuring dataflows with a "file storage" source, such as AWS-S3 or Azure Blob Storage would make the File Format attribute emerge for every single data type, as you should instruct Hunters engine in what what to decode the files coming in.