Types of XDMF readers

We have noticed lately that when we drag our .xdmf files into ParaView 5.6.0 it now presents 3 different readers. XDMF, XDMF3ReaderS and XDMF3ReaderT. What is the difference between all of them? Is there something we can write into our .xdmf file to tell ParaView automatically which one to use?