allan-variance noise terms of MEMS accelerometer

Specific noise terms are given in datasheets of MEMS sensors.
In example the specification of accelerometer contains information about random walk and in some cases the bias instability.

• Both quantities are determined by computing the allan varianz and allan deviation.
  The result of the allan deviation consists out of more noise terms than random walk and bias stability.

  • Why are only random walk and bias instability mentioned in datasheets?
  • Are those the terms that cause most noise?
    If yes why?

• One more thing:
  Different noise errors are represented by different slopes of allan deviation.

• Is it like a characteristic of allan deviation that different slopes occur after calculating a specific amount of overlapping clusters (groups of samples)?

Reference:

• Allan Variance: Noise Analysis for Gyroscopes