Capital allowance - long period

Greetings, I am struggling to understand CA computation on long POA. As i understand, in case of Opening year rules and also in changing the length of POA, the AIA and WDA are pro-rated, but when a trader/company is commencing trading on POA longer than 12 months continuously, CA computation is divided between first 12 months and the rest of the POA?? Is there a difference in CA computation between trader and companies on this matter? thanks