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One reason for numbering things like figures and equations is to refer the reader to them, as in "See Figure 3 for more details."
\label{key}
\label command appearing in ordinary text assigns to the key the
number of the current sectional unit; one appearing inside a numbered
environment assigns that number to the key.
A key can consist of any sequence of letters, digits, or punctuation characters. Upper and lowercase letters are different.
\pageref{key}
\pageref command produces the page number of the place in the text
where the corresponding \label command appears. ie. where
\label{key} appears.
\ref{key}
\ref command produces the number of the sectional unit, equation
number, ... of the corresponding \label command.