When the Integration Service uses multiple threads to read a source file, it creates multiple concurrent connections to the source. The Integration Service may or may not read the rows in a file sequentially.
You can configure multi-threaded reading for direct or indirect file sources in a session:
Reading direct files.
When the Integration Service reads a direct file, it creates multiple reader threads to read the file concurrently. You can configure the Integration Service to read from one or more direct files. For example, if a session reads from two files and you create five partitions, the Integration Service may distribute one file between two partitions and one file between three partitions.
Reading indirect files.
When the Integration Service reads an indirect file, it creates multiple threads to read the file list concurrently. It also creates multiple threads to read the files in the list concurrently. The Integration Service may use more than one thread to read a single file.