Cell Tags From In/Out Names
Cell Tags From In/Out Names gives the selected cell a tag that is the same as its current input or output prompt.
Ordinarily, Mathematica's In:= and Out= prompts (CellLabels) disappear when you close a notebook. This is because the numbers associated with input and output cells are only relevant during a particular Mathematica session.
If you want to preserve the In/Out prompts in your notebook, perhaps for publication reasons, you can make them into cell tags and then show the cell tags.
This command also allows you to locate particular In/Out lines by using the Cell Tags submenu.