When employee receives arrears of salary for the previous years, which was not taxed earlier on due basis, then salary received is taxed on receipt basis .
Arrears of salary are common for government employees as their salary get revised with every pay commission order.Advance salary received is always taxed on receipt basis.
A tax payer can claim relief u/s 89(1) for arrears or advance salary for which he should file form 10 E.