ArcGIS for Server vs. ArcGIS spatial data server

Discussion created by gkar68 on Feb 25, 2013
Latest reply on Feb 26, 2013 by mboeringa2010
I just found that there is a product called ArcGIS spatial data server.
And reading about it, found it is very similar to ArcGIS for server (just subset of it).
So, what is the difference between the two?
is it just a subset of the full server, so maybe it is cheaper?