I'll just try to clarify a little more as it is getting a little complex as we update various builds. In short, buying a product gets you a fixed set of features and content. A membership that includes Trainz Plus gets you regular content and feature updates.
You can buy versions of Trainz from the store (or Steam or Mac App Store etc). TRS19 for example was a product that was released in 2018. It has various built-in routes and DLC packages depending on which version you bought (Regional has less content, Platinum lots more content). Generally features do not change greatly in these versions but Service packs are released primarily to fix bugs.
Trainz Plus is the latest version of Trainz and includes additional features. It is updated regularly. The features and content available in your build are enabled based upon your MyTrainz username and your membership level.
There are times where the store version build number and Trainz Plus build number are the same. There are times where they are not. Again, it's your username and membership level that determines features and content
TRS19 SP5 will not have any new features and will not have a "Trainz Plus" mode (since it isn't the latest version available). It will therefore operate like TRS19 Platinum for Trainz Plus users.
TRS22 will be released in March (and you can get the beta right now). It has brand new content and several new features from Trainz Plus such as UDS, MPS (coming in January), new camera modes etc.
Trainz Plus users who update to the latest build (currently build 111985 ) get "Trainz Plus" features, which right now means Surveyor 2.0 and MPS. Then they'll get more new features will be included as time goes on.