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The sign accompanying the PLSS. It reads

Portable Life Support System
PLSS

To provide a suitable protective environment during both lunar surface and free space excursions, the astronaut carried on his back a compact assembly of various environmental control devices, which formed the portable life support system (PLSS).

The PLSS supplied breathing oxygen; controlled suit pressure; recycled oxygen by removing carbon dioxide, odors, moisture, and some trace contaminant gases; controlled temperature; provided warnings of system malfunctions; and provided voice communications and telemetry data.

The portable life support system supplied oxygen to the pressure garment assembly and cooling water to the liquid cooling garment. The PLSS subsystems were an oxygen ventilating circuit, a primary oxygen subsystem, a liquid transport loop, a feedwater loop, and an electrical power subsystem.

A separate emergency system provided oxygen for breathing, suit pressure, and cooling in case of PLSS failure. The system was called the oxygen purge system (OPS) and was manually activated.