if you use SourceEstimate morphing you make a source space for the
individual subject
you consider based on this cortex mesh. then you get a source estimate you
need to
transport / morph to a template brain eg fsaverage.
if you use source space morphing then you use the mapping between fsaverage
and the
subject to make a source space that has a definition in the fsaverage brain.
it makes subsequent source estimate morphing unecessary.