What is the difference between an Unrestricted Line officer and a Restricted Line officer in the Navy? What is an officer of the Line anyway?

SN Greg Wright Short Answer, unrestricted line officers can serve as Commanding Officer of warships ships.

An unrestricted line officer (in the most general and concise sense) is one whose designator (community identification code) places them in line for command of a warfare asset. For example, ships, ashore, squadrons, etc. Restricted line officers are still officers due proper respect and courtesy, but (again, generally) not in line for command (of warfare assets...I believe they can hold "command" of non-warfare units). There's a third distinction, Restricted Line-Special Duty, generally LDOs.

I'm probably leaving out lots of details that our LDO, RL members can elaborate upon, but I think the best examples of RL would be JAG, Medical, HR, etc while LDOs are usually "Subject Matter Experts" in fields like maintenance and logistics.

The visible difference is the use of a "star" (line officer) or the community emblem (RL/LDO) on the SDB uniform, immediately above the sleeve rank.

As I recall, the term "officer of the line" is a hold-over from when ships fought in "the line" formation. Capital ships were known as "Ships of the Line", so the direct inference to being able to command one.

Unrestricted=no restrictions to commanding a ship. Officer of the line (for all services) are the operational (front line) officers, and not the restricted or support officers. That was what I was taught a few years ago....