Some versions of RHEL, but not all (for example, 7.4), have ISO download options of both a "Binary DVD" (e.g. Red Hat Enterprise Linux 7.4 Binary DVD, 3.78GB) and an "Update Binary DVD" (e.g. Red Hat Enterprise Linux 7.4 Update Binary DVD (20180125), 4.07GB).
I can't find any documentation to explain the difference. It would make sense that the update binary DVD was a point release of 7.4 and used to upgrade from the main 7.4 release, but the dates on this particular version are 2018-03-27 for the Binary DVD, and 2018-03-23 for the Update Binary DVD. That is, the non-update DVD is newer than the update one. Plus the update DVD ISO is bigger in size which would not be expected for a simple patch update.
What is the difference between these two ISOs, and is there a restriction on what each can be used for (i.e. only for fresh installations, or only for upgrades from a particular version)?