Hello, I would like to ask you some questions about Complementary Filter

First, I got this code

angle = alpha *(angle+gyro*dt) + (1-alpha)*acc

and "alpha" is 0.98 which is come from

alpha = T/(T+dt)

and "T" is time constant, so if alpha = 0.98 I'll get T = 0.49 sec

1. I want to know which parameter came first alpha or T ?

(we have to define alpha (0.98) then calculate T (0.49) ,or define T then calculate alpha)

2. How to define "alpha" or "time constant"?

3. Must all units of "angle", "gyro", and "acc" be DEGREE (not Radian) ?

4. What maximum and minimum values of "angle", "gyro", and "acc" should be for plotting graph?