Smart In/Out Cell Labels
Input and output cells have long had labels indicating the order of evaluation. New in Version 12, smart labels can show whether the existing labels correspond to the current session.
Performing evaluation assigns cell labels to input and output cells, numbering them in the sequence they were evaluated.
When the kernel session corresponding to an evaluation ends, the cell labels no longer display the line numbering. This indicates that the cells do not reflect the state in a currently running kernel.
A tooltip can be used to determine what the cell label had been.