What is a System paper
The page you linked to provides a PDF named Author Information Pack (at the very top, right under the headline).

In that file, the different paper types are described. In particular, a Systems Paper is defined as follows:


  Widely adopted semantic systems and systems that generate a far above average amount of interest in the Semantic Web community, may be explained in systems papers. Systems papers are recommended to have 6 - 8 pages in double column format.