What does the Multicast transformation do?
The Multicast creates as many copies of the entire data flow as you want. There is very little configuration available – all you can do is add a Name and Description for each output once you have created it by dragging one to a transformation or destination.
According to Jamie Thompson @ SSIS Junkie (in a post about a bug fixed in 2005 SP2), it doesn’t actually create a new data buffer so it doesn’t double up the memory usage – at least until the data is changed. But be aware if Multicasting large data sets you put yourself at risk of consuming a lot of memory.
When would you use the Multicast transformation?
The main use is where you need to perform different operations on the same set of data – the example quoted on MSDN which matches my experience is where you send the data to one destination and a summarised version to another. Another use i’ve had was directing slightly different versions same set of source data to different file locations.