Hi Kasi,
No, WebSEAL is not part of the webserver. It's a reverse proxy that comes with Tivoli Access Manager (TAM) for e-business (another product). Now called Access Manager for Web (v7)
Normally is in front of the Web Servers or can substitute them.
I don't remember well but from version 6.1 you have a license of TAM e-business with WebSphere Application Server Network Deployment license.
WebSphere Authorization (and if you want Authentication) can be externalize in TAM.
You can get a license of TAM only too.
Regards