You are confusing some things...
Logic Pro will need discrete outs for surround mixing, the build in optical out is stereo only in this regard.
This is why you are confused: There is no realtime surround output to AC-3 (Dolby Digital) conversion available within Logic Pro.
You have to use Compressor, part of the Logic Studio bundle, to convert surround files to AC-3.
AC-3 or DTS files can be streamed through an optical digital output connected to a Surround Receiver with the ability to decode the stream to discrete surround channels.
Best,
m.k.beat