An office is a permanent position which exists independently of the person who fills it. It exists as part of a wider constitution of rights and responsibilities and many offices have some degree of public relevance.
An example of a public office holder is a police constable. Company directors are office holders because their powers and duties form part of the constitution of the company and are required by statute. A priest holds an office within the constitution of the church.
The duties and powers of an office holder are part of the office itself, consequently there is no need for a contract, although some office holders do have contracts. The nature of the contract is determined by all the relevant features, including the exercise of control.
Non-executive directors are not likely to be employees because they are not subject to control. Executive directors are generally subject to greater control and will be employees. Sole directors are less likely to enjoy employee status as the board comprises them alone.
Other types of worker include: