AFMTiming of Tax Relief on Capital Allowances

Hi there, I have notes from LSBF which are telling me the following : "Capital allowances can be claimed either in advance (from year zero) or in arrears (from year. ? If the project is to be undertaking at the beginning of an accounting year, capital allowances would be claimed from year one. ? If the project is to be undertaking at the end of an accounting year, capital allowances would be claimed from year zero" I can't seem to make head or tails of this...can you please clarify? Many thanks