You will be treated as a tax resident for a particular Year of Assessment (YA) if you are a:

Singaporean; or PR or Foreigner who stayed/worked in Singapore for 183 days or more in previous year (excludes director of a company).

well, if you're a singaporean or PR but stay less than 183 days in singapore in a calendar year, you may appeal to be treated as a non-resident (if you think you may be able to pay less tax under that scheme).