Final answer:
Professional references, provided by previous employers or coworkers, offer job-related information about candidates to help employers mitigate issues of asymmetric information in the labor market.
Step-by-step explanation:
The sources referred to in the question, such as previous employers or coworkers who can provide job-related information about job candidates, are best described as professional references. Professional references are individuals who can attest to a job candidate's work habits, skills, and experiences from a professional standpoint. This contrasts with other types of references such as personal or character references, which speak to an individual's moral character and personality, or academic references, which pertain to a candidate's capabilities and performance in an educational setting.
In the context of asymmetric information in the labor market, professional references help bridge the gap by providing employers with insights into an employee's previously unobservable attributes like energy level, work ethic, and professional demeanor. This is particularly important when an employer is seeking assurance about a candidate's qualifications and potential performance if hired. References coming from credible sources like previous employers or colleagues add significant value, especially when an employer is considering a candidate for a position.