This main business is decided by the company itself which part is the main business, so which part of the related costs are the main business costs. For example, computer information technology services, company exhibitions and conference services are all main businesses, so these three related costs are all main business costs and can be divided into three sub-accounts.
Because the service industry has no tangible objects like goods for sale and generally has no main business cost, the main business cost of the service industry can be zero. Travel expenses, salary, rent, etc. It's an expense item, not a cost item. The entries are: debit: related expenses; Loan: cash or bank deposit or accounts payable.