I think it's much like how social anxiety, avoidance personality disorder, schizoid personality disorder and ASD have some (often superficial) differences.
Other dxs would probably be oppositional defiance disorder (these children are often viewed as naughty but most of them have either ADHD and/or anxiety about others having control or doing the work set (e.g., they think it will be/it is too difficult and they react to avoid it) - they prefer predictable 'bad' responses) or simply viewed as challenging behaviour that some alongside difficult backgrounds or attachment difficulties.
You often see this outside of PDA.
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Diagnosed with:
Moderate Hearing Loss in 2002.
Autism Spectrum Disorder in August 2015.
ADHD diagnosed in July 2016
Also "probable" dyspraxia/DCD and dyslexia.
Plus a smattering of mental health problems that have now been mostly resolved.