The pressure chart you have to know
A7 will hand you a set of gauge readings and ambient conditions and ask what is wrong. Memorize the four classic patterns: high on both sides equals overcharge or condenser airflow loss; low on both sides equals undercharge or a restriction upstream; high low side and low high side equals a failing compressor; normal pressures with poor cooling equals a blend door or airflow problem.
Refrigerant chemistry and safety
Know the differences between R-134a and R-1234yf cold. Different service ports, different oils (PAG for 134a, POE or the yf-specific PAG for 1234yf), different recovery machines, and different flammability classifications. Cross-contamination questions show up on every version of this test.
Electronic controls of HVAC
Modern climate systems are CAN-bus controlled and use blend door actuators with position feedback. A single failed actuator can throw a temperature complaint that looks like a refrigerant problem. Study how the HVAC module calibrates actuators after a battery disconnect.