This card-shaped device is used mainly for recreation rather than ceremonial or military purposes and can be replicated to accommodate more than one individual. Each codex records a set number of memories that can be transferred into information by touch (Mass Effect 3: Prothean memory shard). Many of these memories can either contain psionic teachings, history, prophecies, geography, and even dreams.
Great Protoss artists create Dream codices that become equivalent to Terran novels. Some users even use these devices to meditate through memories of Aiur. This has led some non-templars into prolonged escapes from reality (Star Trek - Voyager: Synaptic stimulator). As a result, medical experts are needed to shock these individuals out of the memories in order to perform necessary duties (Naruto: Genjutsu).