Under the INSTALL heading of install.html in the 3.1.4 to 3.1.5 Automatic Update package (and probably the others as well), it says:

Updating from stable releases of phpBB 3.1.x

Full package
Changed files
Patch file
Automatic update package
All package types

These lines are links. If you click the "Automatic update package" link, you get this (in part):

The automatic update package will update the board from a given version to the latest version. A number of automatic update files are available, and you should choose the one that corresponds to the version of the board that you are currently running. For example, if your current version is 3.0.13, you need the phpBB-3.0.13_to_3.0.14.zip/tar.bz2 file.(emphasis added)

Obviously, this should be:

he automatic update package will update the board from a given version to the latest version. A number of automatic update files are available, and you should choose the one that corresponds to the version of the board that you are currently running. For example, if your current version is 3.1.3, you need the phpBB-3.1.3_to_3.1.(latest).zip/tar.bz2 file.