Asd diagnosis

Hi, I am new here. My 16 year old son has just been diagnosed with asd after trying for 3 years to get a diagnosis. The letter states he has difficultys due to underlying asd, I don’t understand what they mean by underlying, I have tried contacting the paediatrician but I’ve got to book an appointment now which could take weeks just to explain the letter in more detail. Has anybody else had this?