It's hard to be precise since "SIP Server" does not have a precise definition, but I'll try to help.
The term SIP Server can apply to any network node meant to affect a SIP session. A server can act as SIP User Agent (UA), proxy, or back-to-back UA. Nothing about the way these applications are programmed is implied by the term "SIP Server."
The SIP Servlet API (SSAPI) is a popular technology for implementing SIP applications; therefore you could say that a SIP Servlet container is a type of SIP Server. The SSAPI is specified in [JSR 289|http://jcp.org/en/jsr/detail?id=289] , and is an extension of the familiar servlet model that is also used for programming HTTP applications.
OCCAS is a SIP Servlet container, and therefore also a SIP Server.
Hope this helps,