By default, Spotfire works in WGS84 (World Mercator). A lot of data exists in different datums and projections but Spotfire has no capacity to be able to simultaneously apply datum transformations so that the layers spatially align. This is a serious fault.
I think this is a very important feature to have. It is also confusing that in the current functionality one can set different datums for different layers but nothing is done about it.