What can you claim as capital allowances

Hello

I am confused what you can claim as capital allowances in the first year of buying an asset and then in subsequent years. I understand there is a first year allowance of 50% and then 25% writing down allowance. But I also read you can claim 100% in the first year of the first £50,000 you spend on business equipment, cars and vans each year. I'm confused.

Does anyone know the current rules and are they about to change?

Cheers

David