Integration Services uses transformations to manipulate data during an ETL dataflow. Transformations can be used in [[articles:SSIS Dataflow Task|Data Flow Tasks]], between data source and destination components, or other transformations.

Sometimes sorted data flows can be a key requirement, especially before joining data with the Merge Join transformation. Two, previously sorted data flows can be merged into a single, still sorted one if they have the same sort metadata (the type and length/precision are the same, in the same order in both inputs).

This transformation supports exactly two inputs and a regular output. It does not support an error output.

The Merge is a partially blocking (asynchronous) transformation: the output is copied into new buffers, the input buffers won't be reused.

If sorting is not a requirement, consider using the Union All transformation only. If more than two inputs should have been merged, use a Union All first, then a Sort transformation instead of Merge.

For more detailed information on this transformation, please refer to this MSDN article.
For the complete list of SSIS transformations, check this Wiki article.