Moved from "Shipwright Discussion," since that forum is exclusively for ship models being built for inclusion in the game.
To help clarify practice in the British navy:
Most naval ships were rated. The link provided is a list of ratings, and these ratings originally had everything to do with the captain's and crew's pay structure -- and little to do with the ship's class or armament. By about the mid-17th century, the rating system had started shifting more toward a description of the ship than Admiralty structure.
A ship of the line, however, was one given a purpose in the "line of battle." This was a basic ship formation developed by the Dutch in the late 1630's, but perfected, standardised, and first used systematically by the English in the mid-1650's. This would be the dominant means of doing battle at sea for about the next 350 years.
These two concepts intersect -- again, referring strictly to English practice -- in that only certain rates of ship were given a role in the line of battle. In the early 18th century, that role was afforded to the first four rates of ship, i.e. ships of approx. 50 guns or more. Smaller ships -- fewer than 50 guns -- were NOT given a role in this battle-line, even if they might be present in a battle fulfilling other roles (carrying messages, towing damaged ships, diverting incoming fireships, carrying provisions, etc.).
Thus, the notion of a 5th or 6th rate "ship of the line" is an oxymoron by definition: such small ships would not line up with their larger counterparts and fight broadside-to-broadside with the enemy's line of battle. They were simply 5th or 6th rated ships, plain and simple.
Note that later in the century (circa 1760 and beyond), 4th rate ships were phased out of the British line of battle. Thus for later time periods, the term "4th rate ship of the line" would be just as incongruous as "5th or 6th rate ship of the line." Yet, up to that point 4th rate ships were still expected to face off against ships with more than twice their count of guns in the line of battle -- which means holding out against firepower an order of magnitude greater than that with which they could fight back.
I hope this helps untangle British naval terminology of the era. Other countries organised their ships and line of battle along similar principles, but the details differed greatly; e.g. in many countries, any ship of 80+ guns would be considered a "1st Rate." But for PotBS, we roughly follow the British classification, and consider any ship of 50+ guns a "ship of the line" for game purposes.