IRAS stands for The Inland Revenue Authority of Singapore. IRAS administers and enforces the collection of income tax, goods and services tax (GST, or also known as VAT in some jurisdiction) and various other taxes.

Year of Assessment (YA) is the statutory tax year for which income tax is calculated and charged. Singapore adopts the preceding year basis of taxation for all sources of income.

That is incorrect and is a myth. It is also important to note that the definition of dormant for ACRA and IRAS is different. IRAS definition of dormant is that it does not carry out business and has no income for the whole of the basis period.

A dormant company in fact must still file its income tax return unless it has been granted the waiver of income tax return.

ECI stands for estimated chargeable income. ECI is an estimate of the company’s taxable income (after deducting tax-allowable expenses) for a Year of Assessment (YA).

If your company fits both the following criteria, it does not required to file an ECI for that YA:
For the financial year, annual revenue does not exceed $5 million, AND ECI is NIL for the YA.

You must register for GST if your taxable turnover is:

Under the retrospective view, more than $1 million at the end of the calendar year, or
Under the prospective view, expected to be more than $1 million in the next 12 months