It is said that the symbols used in Reiki were divine inspiration Usui received during his time on Mount Kurama in 1922.
Reliance on symbolism for Reiki was never a rigid requirement of the practice, rather it was a tool, or mechanism, for the practitioners to enhance their clarity and focus thereby opening the channel for a greater flow of energy.
In the long term, such reliance is diminished as the channel naturally becomes clearer and more open.
The Western teaching of Reiki is generally referred to as Level 1, Level 2, and Level 3 Master. The Usui Reiki Ryoho training was Shoden (Elementary/Entry Teachings) after which Reiki practitioners could treat themselves and others locally; Okuden (Inner Teachings) after which Reiki practitioners could additionally treat non-locally, i.e., distant healing; and Shinpiden (Mystery Teachings) after which the new Reiki Master could attune other people to Reiki and teach the three degrees.
During Usui’s time, most practitioners stayed at Shoden level, a few went on to Okuden level over the course of many years, and fewer still went on to Shinpiden level. Having attained Shinpiden level training did not imply the practitioner had reached enlightenment, but that he had attained sufficient mastery to gift other practitioners with attunements and training.
The symbols given to the early Reiki practitioners are the three symbols given to practitioners of Level 2 Reiki, a.k.a. Okuden, meaning Inner Teachings. They are Cho Ku Rei (Power), Sei He Ki (Harmony), and Hon Sha Ze Sho Nen (Connection).
The fourth symbol given to Western practitioners of Level 3 Reiki, a.k.a. Shinpiden, meaning Mystery Teachings, is Dai Ko Myo (Great Shining Light). However, the truth of this being a symbol used in the practice of Reiki is still up for debate. Some believe it to be used solely for the purpose of attunements while others believe it was merely a piece of calligraphy provided to practitioners as a reminder to make enlightenment their ultimate achievement.
Generally when referencing the Reiki Symbols, you will see either the three symbols of Level 2, or four symbols of Level 2 and 3 together. But, there is still another Reiki symbol attributed to the Usui practice that is not often referenced, which is Tam Ah Ra Sha (Balance).
In addition to these three, four, or five symbols of Usui Reiki Ryoho there are a great number of other symbols attributed to Reiki but they have come from other sources from various practitioners.