What is the different in setting center dot vs bracket?

I tried to do the search but perhaps not giving a right description. I wanted to know what is the different in setting the focus in single black dot vs the parenthesis or both will effect on shooting an image that already have autofocus selected at center (single point)?

Btw, I have a used 50d body and I'm usually set aperture priority or P.

Thanks in advance.

The difference is in how the camera meters. The brackets are for partial metering or center weighted (depending on the bracket type) and the dot for spot metering. If you look at page 100 of the manual it explains all the metering modes in detail.

Thanks for the tip. I brought my camera so don't come w manual. Thanks anyway.

here is a link to the pdf of the manual. Enjoy.
http://www.google.com …Kc0QwxPFRVCFiPm​MQ&cad=rja