Employment Status and Agency Workers
A contract for service is not a contract of employment. People with a contract for services are workers rather than employees. A worker can decide whether or not to accept work, and is free to do the same type of work for more than one employer at the same time.
At the same time the employer is not obliged to provide work for a worker.
The law knows various categories of worker. It is important to decide into which category the particular relationship in question falls for both 'internal' and 'external' reasons: the rights of employer and employee will vary from category to category, and so will the employer's responsibility for his employee to third parties.