What does 135 digital SLR camera mean?

This 135 has been said since the film machine era.

In the past, our film machine had two specifications. One is a camera with 24mm*36mm film called 135, and the other is a camera with 120. The film size is much larger, and the internal structure and appearance of the two cameras are also very different.

After entering the digital age, digital camera imaging is to use a digital photosensitive original to imitate the imaging process of film to obtain digital images. In addition, the internal structure and appearance are similar to those of previous film cameras. Digital cameras that follow the structure and appearance of the previous 135 film camera are collectively called 135 SLR digital cameras today.