Not to make too tight a definition, but: "Pro" means someone who uses the device specifically to make money. So a programmer, creative artist, etc. That's why people complain when a new "pro" device lacks the functionality people need to easily generate an income from the device. Some examples:Where is this standard definition of "Pro" located? I'd like to see what "they" say.
Sales reps and lecturers generally like HDMI to present their ideas in auditoriums, and the humble USB stick is still a must.
Journalists and authors like nice keyboards and long battery life when compiling their stories and images. A wafer thin clicky and noisy keyboard might not be OK for them.
Photographers want photorealistic and color correct displays in the field.
Programmers, video editors, audio editors and so forth need memory and processing power.
In a way Apple has let all of these groups down by sacrificing functionality for size. Back on topic, the iPad pro hasn't been a compromise but a success from a tablet point of view, but I don't agree with people who think it's a replacement for a laptop for most professionals. It's an adjunct.