There seems to be a general misunderstanding of what a spectrum is in a STEM context - including a medical context. Hopefully this explanation might help a bit.
I work in STEM research and spectra are used in many different disciplines to identify or classify complex systems with multiple factors into sets that can be understood, and studied, and to clarify both challenges and opportunities.
I've been trying to think of a not to sciencey analogy and this might be a bit daft but the clue is the name. I have no idea whether the following is a thing but hopefully you get the drift.
Happy frogs, sad frogs, and environmental spectra.
A suitable environmental habitat for froggy can be measured by a number of environmental factors. Max and min temperature, humidity, food type, food availability, pond availability, water temperature, prevalence of predators.
We compare three habitats. The first is high humidity, constant warm temps day and night year round, lots of yummy bugs and creepy crawlies to eat, not just ponds but a full on jungle, very few predators.
The second has scorching day times and freezing nights, bone dry, few bugs, no ponds or water, and everything wants to eat eat froggy.
The third has rather cool but relatively constant temps, muggy sometimes but not too often, puddles more than ponds, cold enough in winter to make froggy sleep for a couple of months, and if only the two legged beasts didn't poison the slugs, his tummy would be a lot happier too.
Each of these habitats are clearly identifiable by the their common qualities. Rain forest - very happy froggy. Desert - very dead froggy. Oceanic - froggy has twenty different terms for rain and is weather obsessed.
Each of these habitats have the same parameters that can be used to identify how happy froggy will be. So I'm sure the spectra for the Atacama and the Sahara will both shout, desert, don't go there. The Amazon and the Congo rain forests will be froggy's top holiday spot to get away from very average Northern European or Colorado winters.
The point of this is that all these places have the same traits, just in a different abundance or quantity. It is the abundance of each trait that results in a pattern, called a spectra or spectrum, that identifies a place as rain forest, desert etc.
The is no mild end or severe end, just different patterns.
When applied to neurological disorders or conditions the pattern can be used to describe a particular diagnosis, what it can't necessarily do is identify the specific needs of an individual. Some froggies will be very happy in the jungle, some more so at the bottom of my garden.
Also check mass spectrometry.