I think that tax changes for the most part would be done through the finance bill rather than through the Supply and Appropriation Act. The finance bill has a series of debates and scrutiny around it, almost, one might say, more so than the appropriation act.
As for the extent of discretion of tax expenditures—and many countries have things like tax credits and the like—the decision is, I suppose, whether it's a tax measure or a spending measure. For us it's largely driven by the statistical treatment and whether it constitutes public expenditure under National Accounts or whether it's a form of tax reduction or a negative tax.