As per my opinion Objects contained in main objects of the company and adopted by the company at the time of starting business forms principal business of the Company.
if u treat principal business on the basis of turnover or revenue than everytime there will be new principal business on the basis of contributin of particular segment in revenue or turnover so u have to take a stable definition.
it should be determined on the basis of object clause in MOA. u can also take income tax return to see whether co. showing under head of business income or under any head.