I often come across questions about how PDA (Pathological Demand Avoidance) fits into the autism spectrum, or whether someone can have an ASC (Autism Spectrum Condition) and PDA, or if PDA is really a new name for ODD (Oppositional Defiance Disorder). As the condition gains more recognition, it is to be expected that people will want to know how to fit it into their understanding of autism or behavioural disorders.
The short answer is that PDA is a form of autism. If someone has PDA, then they have an ASC. No autism, then it's not PDA.
Where it gets tricky is that PDA doesn't necessarily present in a way that people expect for an ASC. This is changing as early markers and diagnostic criteria are refined; skilled professionals can see the rigid stereotypes behind the imaginative play, and the subtle deficits in social understanding underneath the sociability and normal-sounding speech. But if someone is looking for obsessions with trains instead of people or role play, or for the introverted refusal to engage, they might not see the autism. They might get hung up on the outbursts and defiance and start seeing letters like ADHD/OCD/ODD instead of the whole person in front of them.
In some cases I think it can take a very skilled observer to see the underlying autistic traits, especially as the person with PDA might be very skilled at mimicry/role play, and have learned to role play "normal".
The core issue with any type of autism is social communication and understanding. There is some interesting research that points to fundamental differences in perception underlying the differences in social thinking, but nobody has figured out yet quite how it all ties together. You can have social skill deficits with attention disorders, and you can have attention problems in sensory processing disorders, and neither one in itself points to autism. It starts to get a little tricky to unpick what difference is causing what behaviour, but along with the sensory and executive function issues in autism, there will be core issues with understanding how the majority population interact with each other. (Not lack of empathy! But I'll get to that another day)
So someone can be non-verbal and autistic, or they can be highly verbal but still autistic because they just don't get how other people use language to convey so much more than the words mean. An autistic person can be so in tune with their inanimate surroundings that they struggle to connect to people around them in the usual ways, or they can be so socially motivated that they are driven to interact with other people and just can't figure out how all the unspoken social rules work, but they are still autistic.
I think the best phrase I've heard yet to explain autism is a failure to code certain social and communication functions. Elizabeth Newson described PDA as involving a failure to code social identity, and therefore social obligations.
Still a core deficit in social and communication skills, so still autism.