What's the "rms" in the formula of rms for CTRL/RMSRollP

Greets, since we were trying to figure out what’s the relationship between RMSRollP and P gain we set, we put the RMSRollP value in the previous time into the formula, the calculation didn’t fit as current rms value.
In that case, we are wondering that

  1. Where the rms update for every time stamps? and How?
  2. What could also be infered from the parameter in CTRL?

The formula path is “libraries/AC_AttitudeControl/ControlMonitor.cpp”
The function is as following,
void AC_AttitudeControl::control_monitor_filter_pid(float value, float &rms)
const float filter_constant = 0.99f;
// we don’t do the sqrt() here as it is quite expensive. That is
// done when reporting a result
rms = filter_constant * rms + (1.0f - filter_constant) * sq(value);
Thanks for all your patience!!