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 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.