What is a System paper As I was going to submit a paper in one of the Elsevier's journal (Web Semantics), it asked me to select the type of paper. The types include "research paper", "system paper!", "ontology paper", "position paper", "survey paper", "briefing paper".

I don't know what are they, and didn't find a document about them except this which doesn't tell much. I doubt if my paper is just a regular or research paper or a system paper. 

In my paper I proposed a computer application which is a software system. however, it provides a solution for a problem. Does this make it 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.