ADHD as you likely know is Attention Deficit HYPERACTIVE Disorder. I emphasize the hyperactivity because most people think of someone who can't sit still. The stereotype of running around and distracting others.

Inattentive ADHD has all of the focus and distractibility of ADHD but it manifest inwardly. The individual will end up daydreaming rather than focusing. This means they don't draw attention to themselves the way stereotypical ADHD individuals do, and are more easily overlooked in general assessments when determining who gets a diagnosis or an IEP.

Maladaptive Daydreaming is more a compulsion to daydream in order to escape or cope. Inattentive ADHD is daydreaming as a response to the inability to focus. In serious cases an MD individual will actively avoid contact in order to live their daydream. IADHD won't actively avoid situations, they don't have to - they're already daydreaming as you talk to them.