How do professionals determine whether a child’s autism is mild, moderate or severe?

I can’t find a description of which particular behaviors separate the different classifications of autism. Is there an actual list the professionals follow to make this decision or is it subjective?

There is nothing in the diagnostic manual used by healthcare professionals designating the severity of the autism, so that would be something being used by a particular professional or clinic as a descriptor. I would then think that it is a non-standardized description indicating something about the magnitude of the impairment.