The IDNT button activates the Collin's Weather Radar
Ground Clutter Suppression function, labelled as GCS in some models. (It stands for IDeNTify ground clutter). It activates an algorithm which utilises the Doppler shift scintillation of rain to identify rain returns, and removes terrain (and light rain) from the display.
The STAB button turns OFF the radar stabilisation. Normally active, it uses inputs from the attitude reference to tilt the radar down when you are climbing, up when you are descending, and up when the radar scans left, down as it scans right in a left turn (and vice versa in a right turn). This allows the TILT knob to display TILT with reference to the horizon. On older aircraft (without IRS), this stabilisation could go a bit cock-eyed, so turning it off would give you a useful display (scanning with reference to the aircraft's axis) until you could get it fixed (you could also use it as a poor-man's artificial horizon!
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