How does the tags work?

[WebDev]

Does anyone know how this tag works & what it really does?

Is it possible to define a property with that, so that I can pass
individual data (e.g. UserId) - and NOT UserGroups (like
SECURITY_PRINCIPAL)?

What I'd like to do with that is passing data (set in the client) to my
EJBs (Server)!

Could maybe someone post a little example?

Thanks in advance!
Wolf