GNU tar man-page mysterious references to METHOD='replace' and METHOD='system'

My tar (GNU tar 1.28) man-page lists the --atime-preserve option, which applies to the time a file is added to the tarball. The full explanation reads thus: --atime-preserve preserve access times on dumped files, either by restoring the times after reading (METHOD='replace'; default) or by not setting the times in the first place (METHOD='system') However, what is exactly meant by METHOD='replace' and METHOD='system' remains unexplained. Can anyone shed a light on this?