Acreage selection clause

What does Acreage selection clause mean in American Law?

The definition of Acreage selection clause in the law of the United States, as defined by the lexicographer Arthur Leff in his legal dictionary is:

A common provision in oil and gas leases. The pattern is that the lessee, for a price, gets a short term lease (60-90 days) on a large tract of land, upon which he then makes geological surveys. The acreage selection clause empowers him thereafter to extend his lease on certain acreage within the larger tract (usually for an additional payment).