Definition of "trading losses" for

In the UK, when filling a CT600 form:

1. What's the meaning of "Trading losses brought forward claimed against profits" (box 4)? Is it, for a previous year, the operating profit (turnover minus cost) or the profit on ordinary activity before taxation (as operating profit but also including gross interest received and other charges)?

1. How many years' losses can I include in this year's account? For example, for this year's account, can I include losses from the previous two years?

Thanks

Praful