Your forum username and password are also your Trainz username and password which gives you access to the DLS and DLC. It also sets which features are locked and unlocked in your copy of the Trainz program.
For example: originally when I upgraded from TRS19 Platinum Edition to Trainz Plus I did not need to download and install the new program, its features were (back then) already embedded in my copy of TRS19 Platinum Edition. In a nutshell, when you start Trainz it will "dial back home" to see what features you are allowed to use based on your MyTrainz account (which is the same as your Forum account) and it will then give you access to those features (if you have upgraded) or not (if you have not upgraded). The system has changed a bit since then but is still, essentially, similar.
It sometimes happens that the two registration can get "out of sync".