The tension over how to define an employee in a burgeoning gig economy was recently on display: A new federal opinion letter tilted things in a pro-employer direction, making it easier to classify someone as an independent contractor, and a court ruling on a California standard tilted them the other. In between, New Jersey reiterated its pro-worker mandates.

The Labor Department released an opinion letter April 29, addressed to an unnamed concern in the gig economy, suggesting that companies...