Oracle Database Discussions

Hello guys,
using Oracle 11g R2 more and more i have checked out the new AWR and its sections.

I have found some section lke this

SQL ordered by Physical Reads (UnOptimized)DB/Inst: SID/sid  Snaps: 20296-202
-> UnOptimized Read Reqs = Physical Read Reqts - Optimized Read Reqs
-> %Opt   - Optimized Reads as percentage of SQL Read Requests
-> %Total - UnOptimized Read Reqs as a percentage of Total UnOptimized Read Reqs
-> Total Physical Read Requests:         151,508
-> Captured SQL account for   25.3% of Total
-> Total UnOptimized Read Requests:         151,508
-> Captured SQL account for   25.3% of Total
-> Total Optimized Read Requests:               1
-> Captured SQL account for    0.0% of Total

What the heck is "Optimized Read Reqs" and "UnOptimized Read Reqs"? These terms are used very often right now in AWR of Oracle 11g R2.

Does anyone know what this term means and how it is defined? Don't find any information on web and documentation.

Thanks guys!