From MP3000 manual;

CRR models are equipped with either 2 or 3
evaporator fans. All models feature 2-speed
motors. The evaporator fans operate continuously
to circulate air inside the container. The fans
operate on high speed for perishable cargo at
setpoints of -9.9 C (14.1 F) and above. At
setpoints of -10 C (14 F) and below, the
evaporator fans operate on low speed for frozen
cargo.
NOTE: If Economy Mode is ON:
Chill Loads: Evaporator fans operate on low
speed when container temperature is in-range.
Frozen Loads: Evaporator fans stop during the
Null mode; controller operates fans on low speed
for 5 minutes every 45 minutes.