When the Integration Service uses one thread to read a file source, it creates one connection to the source. The Integration Service reads the rows in the file or file list sequentially. You can configure single-threaded reading for direct or indirect file sources in a session:
Reading direct files.
You can configure the Integration Service to read from one or more direct files. If you configure the session with more than one direct file, the Integration Service creates a concurrent connection to each file. It does not create multiple connections to a file.
Reading indirect files.
When the Integration Service reads an indirect file, it reads the file list and then reads the files in the list sequentially. If the session has more than one file list, the Integration Service reads the file lists concurrently, and it reads the files in the list sequentially.