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The Chozo security system is a system designed by the Chozo. Implemented on SR388, it was created to contain the Metroids after their unexpected and out-of-control evolution. To prevent the Metroids from escaping or being exploited by ill-intentioned off-world entities, the Chozo split up the population and sealed each area using Chozo Seals and a corrosive purple liquid that is harmful to off-world entities, but not to on-world bioforms. The Seals could only be activated by uploading the DNA of each Metroid in the area, and would drain the purple liquid to allow access to the next area. In addition to the Seals and liquid, the Chozo installed defense robots, such as the Wallfire and Gunzoo, to guard against intruders of their ruined colony.

However, the security system was only partially effective. Some Metroid larvae managed to escape to the Surface of the planet, and were captured by Galactic Federation scientists aboard the Space Research Vessel Marina. These scientists came to the same conclusion that the Chozo did: that the Metroids had momentous potential when used for the good of galactic civilization, but was equally a monstrous threat in evil hands. The captured specimens would be stolen from the Marina by the Space Pirates, setting in motion Samus Aran's Zero Mission. The Federation would later send Samus to SR388 to exterminate the Metroids. Samus gradually began to disable the Chozo security system on SR388 as she progressed in her mission, by collecting the DNA of killed Metroids and uploading it into the Chozo Seals. She also destroyed many Wallfire, Gunzoo and other sentries in her way.

In the original Metroid II: Return of Samus, no such security system existed; the corrosive liquid would drain from later tunnels shortly after all Metroids in the area were killed, which caused an earthquake. The Gunzoo and Wallfires were stated to be rogue sentries in the game's manual.

Metroid: Samus Returns Official Guide[]

Fast-Forward to the Year 20X5
"The Chozo's security system was partly effective. It kept most of the Metroids in stasis deep within the planet, but some Metroid larvae—their most familiar bulbous form—escaped to the planet’s surface, where a group of researchers captured them. The findings matched those of the Chozo: the Metroids had incredible potential if studied and utilized properly, but were also a danger in the wrong hands."
Wallfire (page 20)
"Part of the Chozo security system, Wallfires are fire-spitting machines shaped in the form of Chozo heads. They’re stationary, but they can take a beating. They spit fireballs or lines of flame in fixed locations and at fixed intervals. They don’t have the ability to target hostiles, but they can certainly slow you down if left alone."
Fireball Wallfire (page 95)
"On your way through this room, you encounter another facet of the Chozo's security system: the Fireball Wallfire. Instead of spitting a stream of flame for a set amount of time, these enemies spit balls of fire continuously. Either time your movements to their fireballs, or destroy them outright. Thankfully, unlike the Black Wallfire, these security machines can be destroyed with a handful of missiles. Use this tactic whenever you encounter them."
Gunzoo (page 143)
"The Gunzoo is another member of the Chozo security system. This robotic enemy only moves along the green rail it’s mounted to, so pay close attention to the rail to figure out where you can go to avoid them. A Gunzoo’s laser is its primary method of attack, which it fires in a straight line in an attempt to drag it over or under you and deal damage. Gunzoos are easy to avoid if you don’t stand still while near them. They’re also destructible, but you’re better off avoiding them whenever possible."