BUILT-IN MATHEMATICA SYMBOL
is an option for Cell which specifies whether a label for the cell should be automatically deleted if the contents of the cell are modified or the notebook containing the cell is saved in a file.
Create a notebook with the setting CellLabelAutoDelete->True:
When inputs are evaluated, cell labels are attached to the input and output cells:
If you edit a previously evaluated input cell, its cell label is cleared:
When the notebook is saved, cell labels are automatically deleted:
When the notebook is reopened, no cell labels are present:
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