Assertion means to say
Critical things which management say directly or indirectly about financial data is called assertions.
if there is sales of 100 Crore that means management is asserting that
they are “Accurate”
they are “Complete”
and they are as per “Cutoff” (this year sales are not transfered to next year and next year sales are not included in current year)
above are assertions of transaction (income /expenses)
similar assertions are there for assets/liabilities and notes to accounts