“主营业务成本”账户核算企业主要经营业务而发生的实际成本，借方登记本期发生的销售成本，贷方登记发生的销货退回、销售折让和期末结转“本年利润”的本期销售成本，结转之后无余额。

In relation to company law explain:

(b) the circumstances under which both a private and a public limited company can reduce its capital, and the procedures through which this can be done. (6 marks)