I am a little confused by all of the new file export options.  I typically export to .wav at 48/24.  Now there is a whole list of other options in addition to that.  Not sure what to pick.  What value was it in the past that was decided "under the hood" and worked?

For what purpose are you exporting? Typically, if you´re about to continue mastering the work in SONAR you stick to 32 bits at export. For CD, as you surely know, it's 44,1/16.

...and if you want to give the individual tracks to an external studio for remixing, etc., from what I read in magazines, they prefer to get 44k1/24bit.

48/24 makes not much sense. If you plan to make CDs you need at the end 44 kHz.
Best is working with 96/24 or 44/24.
Working in a project with 48kHz produces often problemes if you plan CDs.
 

I have external projects that require 48kHz at 24 bits.  My Sonar project is set to 48/24 and when I export it there is an extra box asking "Extra Encoding Options" with values: "Signed 16 bit PCM, Signed 24 bit PCM, 32 bit float", etc.  Sonar never used to ask for these options that I can recall.

The very first option in the export dialog under "Files of type" is "Wave". It is the default option as well.
The other Libsndfile choices are provided as alternate formats such as Wave(Microsoft) and WaveEx (wave format extensible).
It looks like you are picking another wave encoder from the list that requires extra data. 
Just stick with the default option at the top of the list for wave files which is what we have always provided.
If you want to customize that list read this topic.

That's exactly what happened.  Picked a different .wav file option.  Thanks for the help.