\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 can consist of any sequence of letters, digits, or
punctuation characters. Upper and lowercase letters are different.
To avoid accidentally creating two labels with the same name, it is common to use labels consisting of a prefix and a suffix separated by a colon. The prefixes conventionally used are
secfor lower-level sectioning commands
Thus, a label for a figure would look like