Tax expenditures would include, for example, refundable tax credits like the GST tax credits, which are not reported in the main estimates, but are reported in the public accounts. One could argue that the tax system is used to determine the eligibility for the tax credit and not to assess the tax liability of that individual, so there's a disconnect between the treatment of tax expenditure for the public accounts and the way they are treated in the main estimates.
On May 10th, 2016. See this statement in context.