What Massachusetts plumbing work does not require a permit/license?

To make a long story short, I'm a 20 yr old apprentice plumber with 3/5 years of my training complete. Saving to buy a house. I want to open a side business for extra cash flow after work and want to know my legal limitations. From what I understand fixture repair and pipe leak repair under 20ft and drain clearing fit my bill. Please don't comment with negative stuff, I already do sidework and I do a lot of the plumbing for my job so I understand code and why code is as such and abide by it whether we pull the permit or not