Do you mean the superscript number "1,2,4" at the end of the "RedHat Enterprise Linux 5" column heading?
They are not update level identifiers: they are references to the footnotes below the table.
Footnotes 1 and 2 are about Xen support: 5.0 originally had limited HP support (no SAN, no HP management agent) for Xen, 5.2 (and presumably higher) has full support without caveats.
Footnote 4 indicates KVM virtualization is supported in RHEL 5.4 and higher.
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