The d { (calc-left-label) command prompts for a string,
then displays that string to the left of every stack entry. If the
entries are left-justified (see section Justification), then they will
appear immediately after the label (unless you specified an origin
greater than the length of the label). If the entries are centered
or right-justified, the label appears on the far left and does not
affect the horizontal position of the stack entry.
Give a blank string (with d { RET) to turn the label off.
The d } (calc-right-label) command similarly adds a
label on the righthand side. It does not affect positioning of
the stack entries unless they are right-justified. Also, if both
a line width and an origin are given in right-justified mode, the
stack entry is justified to the origin and the righthand label is
justified to the line width.
One application of labels would be to add equation numbers to formulas you are manipulating in Calc and then copying into a document (possibly using Embedded Mode). The equations would typically be centered, and the equation numbers would be on the left or right as you prefer.