Refer to figure.
The purpose and function of the TCAS is to alert the flight crew to the presence of other aeroplanes in their vicinity, and where possible provide an escape manoeuvre, should a collision risk arise.

TCAS relies on other aeroplane transponders to indicate their presence, so aeroplanes not equipped with this system will appear transparent.

Traffic Advisories (TAs) exist when an intruder penetrates the outer bubble caution area and is between 45 and 35 seconds from the collision area. TAs appear as solid amber circles on the TCAS display and are accompanied by the synthetic voice saying “Traffic, Traffic”. This is a potential collision threat.

Resolution Advisories (RAs) exist when an intruder penetrates the inner bubble warning area and is between 35 and 15 seconds from the collision area. RAs appear as solid red rectangles on the TCAS display accompanied by various synthetic voice warnings. RAs indicate a serious collision threat.

The TCAS also has feeds from the radio altimeter to modify the RAs received when in close proximity to the ground. There are no instructions and aural alert given at all when the aircraft is below 400 ft AGL, no descent RAs are given below 1000 ft AGL and no increase rate of descent commands below 1450 ft AGL.