It is the central multi-purpose component of a modular system of detonating cords, pyrotechnics and various other explosive charges with many different uses.
[1] The ignition charge of the device is a blend of zirconium, potassium perchlorate, Viton B and graphite, often abbreviated as ZPP.
[3] To provide additional redundancy and thus make the spacecraft more reliable, a double-bridgewire design was utilized, but during development of the Apollo Standard Initiator it was found that the original design responded unfavorably to RF frequencies.
This new design introduced several other changes to improve resistance and give the device a longer shelf life, such as switching the material of the body from 17-4 PH steel to Inconel 718.
[4]After the Apollo program ended, the Initiator was renamed and re-used on other spacecraft, such as the Space Shuttle.