J2EE engine admin vs. portal admin

Hi,

can someone give me a brief discription of the relationship between j2ee users and portal users?

Both use the UME, right? Where does the active UME store initially reside (portal DB? but then, what if you have a j2ee installation without a portal?)?

Are the j2ee engine admin and the portal admin the same? What happens if you switch the active UME store afterwards to a R\3, what happens with the default portal Administrator user, is it replaced by the R\3 Admin?

Regards,

Sebastian