What's difference between viewing angle and reading angle

The viewing angle defines the area that will provide optimum viewing of the LED display. This angle is determined by the points off-center, both horizontal and vertical, where the measured light intensity is fifty percent of that measured directly in front of the LED display. The maximum viewing angle is defined as the vertical and horizontal points where the displayed information can no longer be interpreted correctly by the viewer.

In other words, viewing angle refers to the angle at which brightness or contrast drops below 50% of the maximum. LED display are available with different viewing angles. Depending on your audience, you may prefer a display with a narrow viewing cone (sometimes more appropriate for drive-by traffic) or displays with super-wide (140 degree horizontal) viewing (usually best for sports venues).

In summary:

Viewing angle is the angle at which the LED's brightness is halved.

Reading angle is the angle at which a typical person can adequately read an electronic message center. The reading angle is typically more than the viewing angle.