The objective of PLUS replaces the work done in the Trainz Developer group from the T:ANE era and early TRS2019 era where a much smaller group of users tested new ideas and possibilities with the Trainz products. With PLUS, we see new things that may or may not make it into a product just like with Trainz Dev, but the product is more refined with fewer crashes.
PLUS users get to see new stuff too that will make it into the regular product stream much sooner than a service pack, or product update such as the UDS and spline to texture or something else. These things plus some new stuff such as the new Multiplayer Surveyor (MPS) are seen by us after a private beta. If they ever do get passed on to the rest of the TRS2019 product line, these things won't appear until much later after a new version comes out. The PLUS version also tests betas too for TRS2019 and up. We got to play with SP1 and did a lot of can't ship this yets before the product was released warts and all.
PLUS also comes with benefits for testing, unlike the Trainz Dev. With PLUS we also get to test new DLC that's passed through the beta program and soon(tm) to be released. Here we saw the newer Canadian Routes, those beautiful Chinese steam locomotives, and other nice content long before it was finally released. I wish the content developers listened to us more when we found issues such as those that occurred with the Tidewater Point Rail remake. There are still some outstanding bugs in that which were reported but were not fixed before the release. Hopefully the new TCCP program with glean these out a bit better than we've had so far because it's frustrating to see them when the product is released.
And finally PLUS users receive for free the DLC we tested along with all the DLC content that has been released so far, not counting the ancient stuff that hasn't been updated yet unless it was already purchased with Murchison 2 and Cabon City being the exceptions because those are really, really old.