I can't download the PDF from JEDEC.

http://en.wikipedia.org/wiki/DDR3_SDRAM

Standards like the DDR3-1600G* are listed as optional. What does that mean?

My guess would be that the specification is outside the normal parameters for a module. Ultra low latency or Very high latency do not need to included in the modules preprogrammed settings. You may still overclock/underclock and/or modify latency settings, but it won't fall under a profile settings. It is up to the module manfacturer whether or not they want to include optional JEDEC profiles.

Modules sometimes use profiles other then JEDEC (they also include JEDEC profiles), for example XMP.