The final part of the three-part test under the Massachusetts Independent Contractor Law is that the independent contractor must be regularly engaged in providing similar services to others.
This does not mean the contractor always has to have multiple clients or customers in order for the employer to meet this test. It does mean that the contractor relationship is not supposed to be exclusive, and there should be some track record of the contractor engaging with other customers and being available to do so.
For this reason, if you have a contract that says you will not work for any competitors during the term of your engagement, there is already substantial doubt about whether the employer can meet this test. Similarly, if you have a long term engagement that occupies your time a full 40 hours a week, it may be difficult for the employer to establish that you are regularly engaged in providing similar services to others.