The dictionary defines 'technical' as the following. One can attempt to apply parts of the definition to how it may relate to 'advanced' diving. Some of my thoughts are in
blue.
Of, relating to, or derived from technique.
As one progresses through their dive training and experience, they use this knowledge to enhance skills and develop technique. These are usually conformed techniques of "best practice" tried and proven over time by those who pioneered advanced diving.
Having special skill or practical knowledge especially in a mechanical or scientific field: a technical adviser.
Many skills are needed when diving in complex environments. The cave diver, wreck diver, and ice diver all utilize skills not needed for basic recreational diving. For example, the running of lines in a cave or wreck to facilitate navigation; or the deployment of lift bags to facilitate drift decompression in the open sea during times of swift currents; or the use of a 'tether' line in ice diving. The practical knowledge needed to survive and deal with adversity in these environments goes beyond what is needed for general recreational diving. This knowledge can include but not be limited to physiology, equipment, and procedures.
Used in or peculiar to a specific field or profession; specialized: technical terminology.
'Technical Diving' is considered a niche in the realm of diving, as is underwater photography. The underwater photographer maintains skills and knowledge for their specific niche in diving, as does a diver engaged in overhead environments. This would be termed 'specialized' and requires additional training and many units of experience to be proficient, let alone master.
Belonging or relating to a particular subject: technical expertise.
Of, relating to, or involving the practical, mechanical, or industrial arts or the applied sciences: a technical school.
Abstract or theoretical: a technical analysis.
Of, relating to, or employing the methodology of science; scientific.
The 'technical' diver delves deeper into the science of the human body and the effects of pressure on it. Boyle and Dalton become common names in the practice of understanding gas and it's properties. Calculating 'best mixes' and understanding of decompression 'theory' are the scientific study of the 'technical' diver.
According to principle; formal rather than practical: a technical advantage.
Industrial and mechanical; technological.
Relating to or based on analysis of market indicators, such as trading volume and fluctuations in securities prices, rather than underlying economic conditions such as corporate earnings, inflation, and unemployment: a technical correction in the stock market.