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All commands that define equation labels are part of the `eq' destgroup.
These are \eqdef, \eqdefn, \eqsubdef and
\eqsubdefn (see Equation references). All these commands use
the equation label as the hyperlink label. However, if the equation
label is empty, they make up a (hopefully) unique hyperlink label for
the destination. This label will be used for the link when you refer to
this empty-labeled equation with one of the equation reference macros.
The command \phantomeqlabel is called to generate hyperlink
labels for the empty-labeled equations. By default, it produces the
labels in the format `PHEQnumber', where number comes
from the count register \phantomeqnumber; this count register is
incremented at every empty-labeled equation definition.
The commands \eqref and \eqrefn (see Equation references) form the `eq' linkgroup. These commands take an optional
argument, which, when present, is tied with \reftie to the
equation reference and becomes part of the link. The equation label is
used for the hyperlink label; if the label is empty, the link is for the
label generated for the last empty-labeled equation.