question about draft_pstc_smplv2_24

In some of the schema files in draft_pstc_smplv2_24, there are occurrences of empty sequences (<sequence></sequence>).

 Is this intentional? Some of the XML tools I use are having an issue dealing with this. As best as I can figure out this is legal schema but I would like
to understand the motivation for its use.

Thanks,

prateek