Acquired federal lands

What does Acquired federal lands mean in American Law?

The definition of Acquired federal lands in the law of the United States, as defined by the lexicographer Arthur Leff in his legal dictionary is:

Lands owned by the federal government which were never public lands (also called “lands in the public domain”), or which were conveyed away to private owners and then reacquired by the government. The distinction between “acquired federal lands” and “public lands” is sometimes legally significant, e.g., with respect to the rules and procedures governing their conveyance, opening to mineral exploitation, etc.