In ALS, Astrals and Trilogies are just BiPAPs with internal, swappable rechargeable batteries and a few extra features past home A/C BiPAPs depending on the model. So many but not all PALS end up with one of these (there are a few other brands worldwide) because they want to go outside the home with them.
The portable machines can also be used to support PALS who obtain trachs and people with other diseases that affect breathing, that need machines more than at home, or who may use settings more suitable for someone who cannot contribute to their own breathing at all. Home BiPAPs don't have the horsepower to support the latter.
I am not sure what the second part of your question means, but there is no law against adjusting settings. It's required that settings be specified in the initial rx, but no doctor or RT can possibly know that they are best for someone in advance. It's not like glasses or hearing aids. It's more like throwing a dart.
Past the initial rx, just like using more or less of a drug depending on your symptoms, the settings are in your hands, if you want them to be.
Our system unfortunately is not built for supporting xPAP post-sale, including limited community college enrollment in RT programs. But that is a whole different subject.
Neither I nor anyone who advocates responsible titration of settings believes in random exploration, which could be dangerous, just like deciding to triple-dose Ambien on a whim. Instead, we provide education so that such titration can be performed safely and effectively. It is a progressive disease, and settings should not be set and forget, either in everyday life or emergencies. It's sad that most people know more about how to adjust their microwaves than their respiratory support.
I am always happy to help anyone control their BiPAP. And for those interested, there is a two-page "BiPAP Basics" document on my profile. I am also working on a sticky that will loop in pulmonary testing, but there is no straight line between testing and settings at all.