difference between unrealised rent and arrear of rent

As per new section 25A(1), the amount of rent received in arrears from a tenant or the amount of unrealised rent realised subsequently from a tenant by an assessee shall be deemed to be income from house property in the financial year in which such rent is received or realised, and shall be included in the total income of the assessee under the head “Income from house property”, whether the assessee is the owner of the property or not in that financial year.