Shift abbreviation inserted for days off

What causes this issue?

You have overwritten a shift with 'blank'. As a consequence, the
overwritten shift is deleted. The overwritten shift was an employee's
shift from a substitution or an employee's shift from the work schedule
rule.

System Response

The system inserts the shift abbreviation of the shift group which
represents a day off.

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.