The TDX system, renamed the SanX) in the '90s and then discontinued at least 15 years ago was a combination 9 gallon holding tank/Type I treatment device. The treatment function would not be legal
in "No discharge" waters. But you wouldn't want to use the chemical certified for use with it even if you are in waters where it's legal
to discharge treated waste anyway because it's a nasty formaldehyde based witches brew that may kill off the required amount of bacteria but not ANY of the odor
. I had a TDX on an '85 340 SeaRay, so I know that SeaRay puts the discharge thru-hull above the waterline. I had to make sure I was in open water
well away from any other traffer and running at about 1500 rpms to create wash over the thru-hull 'cuz otherwise the stench drifting back into the cockpit
during the several minutes it took to empty the tank was awful.
So what you actually have a 9 gallon holding tank
that you'll pob'ly want to replace with a larger one. The only light you need to pay any attention to is the level in the tank. When it reaches 3/4 full, it's actually only 1 flush away from full, two away from overflowing out the vent.
If you're also new to the VacuFlush, I've written an article that will help you understand how it works. I'll be glad to send it to you if you'll send me an email
(click on my name at the left of this post to see option to send email
or PM...choose email).