A PLLC is a professional limited liability company. These exist in States that require LLCs composed of State license holders to use this category.
They differ from regular LLCs in that only a license holder (doctor, lawyer, others) may register the company and there may be limitations on ownership. Some States require all of the members have the specific license for the service offered while in others it may be as low as 50% professional ownership.
In all States that require PLLCs, notification and approval of the State regulatory board for that profession must be obtained. For instance, a physician partnering with other doctors in a PLLC would have to get approval from the State Medical Board before filing with the Secretary of State.
Difficulties arise when a licensed member dies, loses their license or wishes to exit the PLLC. In some cases, the company has to be dissolved (and perhaps recreated). Transfer of ownership in a sale may also have restrictions at the State level.