What is the first ITR1 or 10E

Tax is calculated on your total income received during the year. If your total income includes any past dues paid in the current year, you may be worried about paying a higher tax on such arrears (usually tax rates have gone up over the years).

To save you from any additional burden of tax due to delay in receiving income, the tax laws allow a relief under Section 89(1). If you have received any portion of your salary in arrears or in advance, or you have received a family pension in arrears, you are allowed some tax relief under Section 89(1) read along with Rule 21A.

If you have received any portion of your salary in arrears or in advance, or you have received the family pension in arrears, you are allowed some tax relief under section 89(1) read along with Rule 21A.

Here’s how you can calculate the tax relief yourself –

Step 1: Calculate tax payable on the total income, including additional salary – in the year it is received.

Arrears provided for will reflect in Part B of Form 16.

Step 2: Calculate tax payable on the total income, excluding additional salary in the year it is received. You can get the amount of the additional salary (Arrears) from the arrear document given by your employer.

Now you have to subtract the arrear from the total salary received (including the arrears), which can be taken from your Form 16.

After knowing the amount after arrear, you need to calculate the tax over the same.

Step 3: Calculate the difference between Step 1 and Step 2.

Step 4: Calculate tax payable on the total income of the year to which the arrears relate, excluding arrears.

Step 5: Calculate tax payable on the total income of the year to which the arrears relate, including arrears

Step 6: Calculate the difference between Step 4 and Step 5.

Step 7: Excess of amount at Step 3 over Step 6 is the tax relief that shall be allowed.

If the amount in Step 6 is more than the amount in Step 3 no relief shall be allowed.

Alternatively, you may follow the steps on the income tax website to calculate the tax the arrears.

To claim the benefits under section 89(1), filing of Form 10E is mandatory. This form can be filed online on the income tax e-filing portal. To access the form you must login to your account.

Both should be simultaneous; anyone can be first, followed by another.

1. Generally, ITR does not require to provide any proof for claiming deductions, exemptions or relief for that matter. 
2. However, for claiming relief under sec 89, the IT department has made it mandatory for you to file the necessary details in Form 10E. So I recommend you to first fill form 10E where you have rightfully claimed your relief (just like making necessary investments in sec 80C before the end of the FY ) and then claim the necessary relief in ITR. 
Please correct me if the above interpretation has an alternative view.