There are humans on a number of worlds that are borderline in terms of survival--high/low temperature, oxygen, water, gravity, pressure, radiation, and the like. A variety of ways have been used to deal with that.
One of them is protective gear and facilities for skilled corporate employees and colonists. Although this does not solve the long-term problem of spreading human life to more habitats, it allows the short-term exploitation of rare resources.
Long-term solutions involve mechanical or genetic augmentation that is either reversible, permanent, or hereditary. Human augments and mods whose permanent or hereditary modifications cause them to diverge from humans both in their livable environments and behaviors are sometimes called abhuman, and are either privileged or discriminated against because of it.
There is a Fleet enforced prohibition from making "servile species," especially from human base materials, but some star systems and corporations have been known to operate in gray areas where a combination of educational, social, genetic, and financial factors combine to keep a population captive.