What's a "minimum" security prison?

Can't they call it "Low security" like in the UK. I propose if security was at a minimum, there wouldn't be any at adjectives and prisoners could walk free.

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In America, there are currently four classifications of prisons. Minimum, or the lowest. Some of these resemble college campuses, and the inmates are almost on an honor system. Medium, these own walls, and some restraints built into them. Maximum, these are usually the ones that are for initial entry into the corrections system, where classification takes place. Super Max. These are reserved for prisoners, who can not live in a run of the mill prison population, as they are too violent.

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