Generally to say, treated fibers are those that have a flame retardant chemical that’s applied during the fiber forming process. As a result, it makes the fibers, flame resistant fibers. Fabrics made from treated fibers are flame resistant for the life of the garment.
One fiber type is a treated 100% rayon. These fibers get treated in the fiber forming process and are flame resistant forever.
The flame retardant chemical can't be removed by normal wear or laundering. The garment would no longer be flame resistant only if it becomes torn or soiled to the degree that the soil won’t wash out.
Another fiber type would be a blend of cotton and Modacrylic fibers. Fabrics made of these fiber blends are characterized to have a soft and comfortable cotton-like hand. The Modacrylic fiber that gets added has soft and strong components. It’s also resistant to chemicals and solvents. This makes these fiber types ideal in flame resistant environments.
Applications for these fiber types are a bit more general. Industrial protective clothing, utilities and fire fighter work uniforms are good matches. Like treated fabrics, it’s recommended that treated fibers get washed the same way.
Water that is too hard could leave deposits that could ignite if exposed to an ignition source. The only significant difference between caring for the two types is that the Modacrylic /cotton blends should be treated in soft water with non-chlorine bleach as it weakens the fabric.