To maintain the Principle of Natural justice, the delinquent employee is served with a notice justifying the charges of his/her offence/misconduct and asking for explanation or to present his/her stand on the charges within a reasonable time so that he/she is given reasonably a fair chance to present his/her case is called show cause notice.
eg. An employee remains absent from duties frequency without prior sanction or intimation and without valid reasons or remaining absent from duties without intimation for more than 10 consequtive days, he is liable for disciplinary action under applicable standing orders.
Assume he's been issued with warning letters for this misconduct many times earlier, still he is not improving, then he may be served with SCN asking why appropriate disciplinary action cannot be initiated against him. In some cases, there can be 1st SCN, 2nd SCN & Final SCN so as to provide ample of chances for improvement. Still no improvement is seen, then the employer can initiate appropriate action against him.

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Surabh