ITR-1 cannot be filed by any individual who:

is a Resident Not Ordinarily Resident (RNOR), and Non-Resident Indian (NRI)
1. has total income exceeding ₹ 50 lakh
2. has agricultural income exceeding ₹ 5000/-
3. has income from lottery, racehorses, legal gambling etc.
4. has taxable capital gains (short term and long term)
5. has invested in unlisted equity shares
6. has income from business or profession
7. is a Director in a company
8. has tax deduction under section 194N of Income Tax Act
9. has deferred income tax on ESOP received from employer being an eligible start-up
10. owns and has income from more than one house property
11. is not covered under the eligibility conditions for ITR-1

Following are the types of income that shall not form part of ITR 1 form:-

1. Profits and gains from business and professions
2. Capital gains
3. Income from more than one house property
4. Income under the head other sources which is of following nature
   winnings from lottery
   Activity of owning and maintaining race horses
   Income taxable at special rates under section 115BBDA or section 115BBE
5. Income to be apportioned in accordance with provisions of section 5A

Yes, it is mandatory to define the nature of employment while filing of return from the following:

Central Government Employee
State Government Employee
Employee of Public Sector Enterprise (whether Central or State Government)
Pensioners (CG/SG/PSU/OTHER)
Employee of Private Sector concern
Not applicable (in case of family pension income)

You would need to download AIS and keep copies of Form 16, house rent receipt (if applicable), investment payment premium receipts (if applicable). However, ​​ITRs are annexure-less forms, so you are not required to attach any document (like proof of investment, TDS certificates) along with your return (whether filed manually or electronically). However, you need to keep these documents for situations where they need to be produced before tax authorities such as assessment, inquiry, etc.

• Download AIS and Form 26AS and check the actual TDS / TCS / tax paid. If you see any discrepancy, you should reconcile it with the Employer / Tax Deductor / Bank.
• Compile and carefully study the documents to be referred to when filing your ITR, like bank statement / passbook, interest certificates, receipts to claim exemptions or deductions, Form 16, Form 26AS (Annual Information Statement), investment proofs, etc.
• Ensure details like PAN, permanent address, contact details, bank account details, etc. are correct in the pre-filled data.
• Identify the correct return for you (from ITR-1 to ITR-7). Provide all the details in the return such as total income, deductions (if any), interest (if any), taxes paid / collected (if any), etc. No documents are to be attached along with ITR-1.
• e-File the return of income on or before the due date. The consequences of delay in filing returns include late filing fees, losses not getting carried forward, deductions and exemptions not being available.
• After e-Filing the return, e-Verify it. If you want to manually verify your return, send the signed physical copy of ITR-V Acknowledgement (by speed post) within appropriate timelines of filing the return to Centralized Processing Center, Income Tax Department, Bengaluru 560500 (Karnataka).

Different tax returns are prescribed for filing by individual taxpayers depending on their source of income and residential status. To determine the correct ITR to file, you can use the Help me decide which ITR Form to file option. You can then proceed to answer the questions displayed to you to determine the correct ITR form to file.