If the customer works, they will usually have one of three occupation status.
Self-employed/Sole Trader - These customers sometimes get classed as sub contractors. They file there own tax and do not have a limited company and work for themselves. They might have a business name, (example) Joe Bloggs Trading as Joes window cleaning.
Employed - These customers work directly for a business and are on the companies books as an employee. They are usually initialed to benefits such as holiday and sick pay.
Company Director - These customers have there own business. The business should be findable on companies house. Find and update company information - GOV.UK a company director may opt to insurer and register a vehicle in the company name for tax relief purposes.
Other statues may come up such as -
R = retired
U = Unemployed
H = Houseperson/Housewife
E = In fulltime education