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One of the sign accompanying the IU. It reads.

The Brains of the Saturn V Rocket

The Instrument Unit, or the "IU" Ring, contains the equipment which guides, navigates, and controls the Saturn Launch Vehicle

The IU ring is positioned atop the rocket's upper stage just beneath the Lunar Module. A digital computer within the ring calculates the proper course, engine ignition, stage separation, and telemetry.

On a lunar flight, the IU calculates the proper course to the Moon and issues steering commands to the third stage. Steering is done by gimbaling, or moving, the engine. It also keeps the Saturn's upper stage in a stable position while the crew members are docking the command module.

The IU ring was designed by Marshall Space Flight Center and was fabricated, assembled, and tested by IBM at the Space Systems Center in Huntsville.

While it is true that the IU is located mostly underneath the LM, some of the LM actually extends below the SLA, into the empty space in the center of the IU, and even around the periphery of the S-IVB tank dome.