Supplementary estimates set out proposals for amending the departmental spending the House has previously authorised via the main estimates. They are used if departments need to spend more (or significantly less) money during the financial year than the House has agreed they can—for example, because they have taken on new functions.
Supplementary estimates are normally presented in February each year and approved in March, before the main estimates for the following year are published. Not every department will publish a supplementary estimate, as not every department will require additional funds.
You have the opportunity to debate the supplementary estimates on an estimates day.