Trainz+ beta may have a newer build number than the current-released version. These are the beta versions and it's done so that users using the test-version can't upload content to the DLS - this causes the DLS to reject the upload. Beta versions not only change quickly but also have bugs in them that are worse than what may already exist which is why they don't want content uploaded.
Unless the beta shows to be absolutely amazing, always keep the current-release and the test copy separate and never overwrite your current working copy, and even if you plan on using the beta version as your working-version, backup your data first. I gone as far as to replicate my data and placed that on a separate hard drive and use that copy as the beta database to ensure that nothing cross-pollinated unexpectedly due to updating the wrong version.
Once the testing is done and the product is updated, the other products are updated to the latest build number and build version, and uploads are allowed. This has been the MO for some time.
As pware said, they're all based on the same code. The different features are enabled based on your license. This isn't uncommon today with software. Things that work in Plus should work in other versions of TRS22, unless there are specific features such as the new Trainz Living Railway (TLR) or a route using HD terrain. HD terrain works fine in PE but not regular TRS22.