All income derived by an employee from his employment are taxable unless specifically exempted under the Income Tax Act or covered by an existing administrative concession. Refer to here

Whos is treated as a tax resident for a particular Year of Assessment (YA) :
 - Singapore Citizen (SC) or Singapore Permanent Resident (SPR) who resides in
   Singapore except for temporary absences; or
 - Foreigner who has stayed / worked in Singapore (excludes director of a company)
   for 183 days or more in the previous year. i.e. the year before the YA.

Year of Assessment (YA) refer to the year in which income tax is calculated and charged. The assessment is for the income earned in the preceding year, starting on 1 jan and ending on 31 Dec

Example : For YA 2018, the assessment is for income earned from Jan to Dec 2017

Assessable income is total income after deducting allowable expenses and approved donations. 

Personal reliefs are the deductions which help you to save tax. Tax reliefs and rebates are allowable if:
- Singapore tax resident; and
- meet the qualifying conditions.

Chargeable income is assessable income after deducting personal reliefs.

Source :
https://www.iras.gov.sg

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