Refer to figure.
Modern aeroplanes are fitted with equipment that provides collision avoidance assistance. TCAS, which is an abbreviation for Traffic Alert and Collision Avoidance System, is the current equipment used to achieve this function, and provides traffic information within approximately 30 miles of the aeroplane.

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 II detects intruders in the TCAS aircraft’s vicinity, assesses the collision risk and presents warnings to the crew in the form of TAs and Resolution Advisories (RAs).

RAs offer manoeuvring advice in the vertical plane to resolve conflict. If your aircraft and the intruder both have Mode S data link transponders the system will co-ordinate the RAs to provide complementary vertical avoidance instructions.

INFORMATION DISPLAY
The action required to comply with the Resolution Advisory displays on a specialised Resolution Advisory/Vertical Speed Indicator (RA /VSI) instrument using a colour liquid crystal screen, which shows the rate of climb, and intruder information. On other aeroplanes, the intruder information displays on the EFIS Navigation Display screen.

In either case, the symbology used is the same:
Hollow White or blue diamond - Other traffic not offering a threat.
Solid White of blue diamond - Traffic within 6 nm and 1200 ft vertically.
Solid Yellow Circle - Traffic advisory.
Solid Red Square - Resolution advisory.

Each symbol has a data tag attached to it, which appears in the same colour as the associated symbols.

This data provides the altitude of the target in reference to your own aeroplane’s altitude (e.g. + 06 indicates 600 ft above). An arrow also indicates whether the ‘target’ is climbing or descending.

There is also often provision to display momentarily absolute pressure altitude in hundreds of feet of pressure altitude.

Where the target is a mode A transponder with no height transmissions, no data tag appears and no RA is given. If the target does not have a functioning transponder, the TCAS system is unable to detect its presence.