This is such a feature rich product.
I use Pc99 all the time whenever I transfer disks from TI to the Pc or the other way. It only requires a standard RS232 interface. No modifications to the RS232 are required in contrast to what others wrote!
I use USB on Pc side.
I admit, I aquired Pc99 Light because I wanted to get the Cyc. And the deal was totally worth it.
The Cyc contains more than 70.000 pdf pages (searchable text, not a scan or bad OCR), in 1641 pdf files.
It includes plenty of internal TI documents you won't find on whtech or other pages.
It includes 3339 disk images.
The Cyc main document is an Index, in 2015 this was 1860 pages large!
This is the root of every search you are doing, you want to know about who Jeff White is and what he did? Go to the Index. You will get a short bio and all public references.
You want to know about the Cortex? Go to the Index. It will link you to all public references. And you have those references in full text on your Cyc.
The GEnie File List (another pdf) alone is 4046 pages large.
You get all PHM, PHD, PHT manuals with all cartridge binaries and disk images.
You get the Control Data Console Files, the v2.2, the 99/4, the 99/4a and the OPA SOB console.
You get so many technical documentation, not just from TI but from Myarc, Mechatronic, MillerGraphics, Corcomp...
You get the Cartridge Collectors Guide (302 pages) from Bill Gaskill, you get a 122 pages edition with photos of his TImeline.
You get 43 complete books in full text (excluding technical data books here) and a 476 pages book list.
You get all magazines with their disks (if there were), updated with fixes and cross referenced.
You get 214 pdfs from Usergroups (mostly Newsletters), some of those files include the whole newsletter archive of a usergroup.
You have a licensed edition of all Disks/Modules/Roms with the suite.
Will include some feedback on the Pc99w once I have it.
Regarding Pc99 Dos Feature Set:
You can download the manual for Pc99 Dos (former Pc99 Stage6), it includes a Feature list for sure: