You will not want to use substream to send to another application - the video quality will not be as good and it will miss-read plates.
I believe you can stream the mainstream to 3 devices at the same time.
Going off your other thread, I encourage you to try the trial that Plate Recognizer offers - simply create an account and they give you a token number and you put that into Blue Iris and you start getting plates with the plate number placed on the alert video and in the BI log.
Actually try Plate Recognizer and OpenALPR trials and see which one you like better - some here like OpenALPR and others like Plate Recognizer.
That is why I like Plate Recognizer because it logs the plates in the BI log text file so it retains the data until you delete the log file, plus you can simply take the file at the end of the month and import into excel and sort and delete all the entries but the plates and then do sorts and stuff.
Updated 2021.08.20 Both Downloader and Viewer have been updated. Most changes relate to being able to pull from the local web-server as well as the cloud CSV to allow much faster imports and thus alerts. Tied into this improvements made to alerting, searching, backups, etc. Please see This...