What does it mean to have a camera of several hundred d among some professional photographers? I am a novice. Please explain.

500D, 550D, 600D, these are the models of Canon SLR cameras (specifically, the body models of SLR cameras), and they do not represent any parameters of the camera.

Canon SLR camera is named "number+letter D", and D stands for digital SLR. The fewer digits, the higher the end. Single digits are full-width SLR and flagship half-width SLR, two digits are mid-range SLR, three digits are low-end SLR, and four digits are the lowest-end entry SLR.

Nikon SLR is named "letter D+ number". The meaning of D is the same as that of Canon, and the meaning of number is a bit confusing. The single digit is the highest-end full-width SLR, the three digits are the full-width SLR and the flagship half-width SLR, and the two digits are the mid-range half-width SLR, but they have not been upgraded according to the two digits (probably D90 found the road blocked and refused to use the D92 and D95 models). A new four-digit model.

The number of sets with lenses should not exceed 4000. Nikon D3 100, D3200 and D5 100 are optional. The D5 100 sensor is recommended for higher grade, and it is the same as the mid-range D7000.