TTS-1 does not have a separate installer.

And don't forget there's also Rapture Pro, but that's a paid upgrade.....

Yes as Steve said, TTS-1 is built right in to all versions of Sonar and this goes wayyyy back in time. It is not a VST so does not require installing separately. It is a DXi synth and seems to be needing an update to run smoothly these days, it's becoming a little buggy on Platinum at least. 
 
It's still a very good sound source and some of the sounds are as good as anything else out there. I hope they keep it going.
A trick Steve taught me that's a huge time saver is if you uncheck all midi outputs in preferences TTS-1 will automatically load up when you OPEN any midi file. Makes it super easy to preview midi song files.  

I found this description for Rapture Sessions in the November 30, 2015 press release:  "Rapture Session virtual instrument delivers 4 GB of exceptional sounds with an easy-to-use interface".  "It's compatible with sound expansion packs for Rapture Pro, Dimension Pro, Rapture, and Cakewalk Sound Center making it an extremely versatile, useful, and above all, expandable synth".  There's also this product page: https://www.cakewalk.com/Products/Rapture-Session.
 
The key fact for me is it is a 32/64 bit compatible player that replaces 32 bit only Cakewalk Sound Center.

scook I asked Cakewalk Presales about this. Their reply was SONAR Home Studio includes 6 instrument plug-ins: TTS-1, Studio Instruments Suite (Guitar, Drums, Piano and Strings), and Rapture Session. They also said they were forwarding this issue to both the web and documentation groups because both have the instrument list wrong

scook, Will you contact the web and documentation groups to ask when the comparison charts will be updated?  Six months later and the charts are still incorrect.

jimfogle

scook I asked Cakewalk Presales about this. Their reply was SONAR Home Studio includes 6 instrument plug-ins: TTS-1, Studio Instruments Suite (Guitar, Drums, Piano and Strings), and Rapture Session. They also said they were forwarding this issue to both the web and documentation groups because both have the instrument list wrong

scook, Will you contact the web and documentation groups to ask when the comparison charts will be updated?  Six months later and the charts are still incorrect.

We already have a fix in with documentation and a note with Web. All of these changes will start to go out with the slated maintenance update.