Some questions about Complementary Filter

Using Arduino Sensors
1

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?