That section is headed "Is a boat part of a dwelling" rather than "Examples of when a boat becomes a separate domestic hereditament'
Rating is rather dull - so I try to avoid it as much as possible but, as a valuer, do get dragged in occasionally. At its most pure, and it has been meddled with by various governments to suit the policies of the day, it is an annual tax on the occupation of land, according to its value. When Poll Tax was introduced, domestic rating was abandoned (but not business rates) and when Poll Tax was dropped, it was replace with Council Tax, Essentially Council Tax and Business rates are conceptually similar - but CT is assessed on capital value "bands" and business rates of rental value in £ per annum. Insofar as these are both points on which a rate/%/flat charge levied, there isn't much difference. Both are (usually) collected by the local authority - but how the collected sums are retained or passed to other governmental bodies varies.
The Valuation Office (a government agency) is required to maintain Rating Lists in which pretty much every property (land) is listed, one way or another. There are few types that are not assessed - but not many. Each herediatment item has an entry; it is an archaic word mainly reserved for rating although our north-of-the-border friends use it more widely. As a property (i.e. land) tax, there cannot be an assessment for, say, a boat or caravan if not annexed to land - even though they might be commonly used as a dwelling, either part of full time.
When it comes to assessing the value of land - in order to calculate the amount of tax - then this includes what is annexed to the land. Most obviously, a house will usually increase the value of the land. But when it comes to boats and caravans, it becomes more difficult to know whether they are annexed or not. Two moorings, each of which will be assessed for (say) Council Tax, could have identical boats at any point in time - but one of which is only staying for one night and the other never moves. The one with the boat annexed to it with have a higher banding. Naturally, it becomes a bit subjective as to how-annexed-is-annexed - hence the guidance. It also seems a but odd that you can have a Council tax assessed mooring, but no boat is present. But it is not that different from a house remaining a house, even though there may be no-one living in it.
The composite hereditament is something else. When, following the abandonment of the Poll Tax, there emerged a domestic and a non-domestic property list, it became necessary to put property into one or another. But, for example, a pub where the living accommodation shares a kitchen with the commercial side, it was necessary to ensure the kitchen was split. It's a bit of a bodge, but it is said to be a "composite" property and valued accordingly.
Moving back to boats, a leisure marina will be be assessed for business rates. But if a few moorings become domestic (regardless of whether individual boats are annexed, or not) - they will either separately or collectively be assessed for Council Tax - and removed from Business Rates. If the mooring(s) are very largely independent of the other facilities - then no problem. But more typically they will share the leisure facilities such a car park, maybe pontoons, services etc - so the moorings become "composite" . So in a marina it might well be possible to have a mixture of composite (sharing the leisure facilities), non-composite (not sharing the leisure facilities) and business rated (leisure) moorings.
Whilst on the subject, VO can sometimes be persuaded that, say, a pontoon is the hereditament as the various, residential boats shuffle around it. The banding will be higher - but the Council Tax for the highest band is only three times that of the lowest band there can be a significant saving overall.
And planning permission is not essential to being assessed to these taxes. It might well be a factor in being spotted - and possibly in deciding whether you have created a new hereditament and at what value - but no PP does not automatically mean not property tax.
I said it was dull.