BSP Directives

There is a new (to me) BSP directive in 6.20, namely

<%# %>

However, there is no documentation! (I looked in 6.20 and 6.30 help.sap.com)

So, what is it? While I have your attention, where will I ever find documentation on BSP Extensions, such as phtmlb? These things look very interesting, but with no documentation I will stay far away from them.