* how is the existence of any category intended to help the person/people using it?
And therein lies a big part of the many variations and opinions. Different WikiTreers, and sometimes the same wikitreer use these shipping categories for mulitple purposes often depending upon the information to be found in the shipping records for the particular voyages they are interested in. What's available varies widely with time and part of the world.
For example, I have most often used shipping information in regard to the Danish lines I'm researching. There I've found that people who knew each other in Denmark or who traveled together on leaving Denmark often settled together or nearby in the United States. In trying to determine if a Nels Jensen I found in the U.S. was the one I was looking for, the fact that a Fodor Andersen was a passenger on the same ship sailing with the elder sister of my Nels Jensen and that a Fodor Andersen appeared close to a Nels Jensen in the Kansas census 15 years later made it much more likely that I had found the right Nels Jensen in Kansas. This use doesn't really depend upon any familial relationship between people on the same ship. Its a clue I would not have been aware of if they had not appeared together.
Another way that I use the information is in looking for groups of people traveling together who may be related. If I can match a group of people on the same sailing with multiple names appearing in the same household on a Danish census of a particular place, then I may have found a familial relationship in a group of people immigrating and can try to find the family group after arrival in the destination place.
A third use is to help in keeping track of who in a particular sailing group has been added to WikiTree and who still needs to be added.
Add to these different uses that there is likely different information available depending on how I approach shipping. For example, if working from the Danish Emigration database, often the specific ship name is not listed. Dates are given and all the people traveling on the same contract are listed. Also some location information in Denmark is listed. But ship names and arrival dates have to be figured out by correlating with other information like who else traveled on the same date or close contract numbers.
On the other hand if I'm working from the arrival shipping lists in the United States, ship names and the arrival date are usually available, but its much harder to figure out groupings of passengers into a single party as there are no family groupings or groupings by contract number and usually no information on town of origin or sailing date.
I've also worked with 17th century ships from France to Quebec and they are a whole different kettle of fish in terms of the information available and what can be done with it. Usually there are no documents like the 19th century shipping lists for arrival in the United States. We are lucky to find an account of a particular ship's arrival in a letter in the Jesuit Journals sent back to France or a mention that a group of people baptized included a group of men (often listed individually) who arrived on x ship the month before in a priest's report in the church records. Here the category may eventually help us put together a sort of passenger list from a variety of different documents.
It is still likely that people traveling together were either already related or often later became related by marriages and settled together. But without documents like ship passenger lists with standardized information, its much harder to establish information like firm departure and arrival dates. We are lucky to figure out a year and maybe a season of the year. But still connecting people to a particular voyage means they knew each other in a fairly small group over a period of many weeks and probably months. Out of this often grew the links of community that lasted into the new settlement and often developed into familial relationships if they did not exist before.
My point is that both the information available and the ways it can be used vary far too much to think that we can mash it all to fit in a particular mold at the landing level where there should be one category for each voyage. The best we can hope for there is a sort of pattern that puts a lot of elements in an order and grammar type where we can use the available elements to create a category name. There will be a little bit of a pattern, but it will vary a lot because which elements are available to be part of the landing level category name will vary from voyage to voyage. Groups of voyages that fit within a particular project are likely to have similar information available and so some projects will be able to define a good landing level pattern for the voyages they work with. But they likely won't be like the pattern for voyages in other times or parts of the world.
But since we can have multiple paths in the upper category structure and not all the upper paths must be used for each landing category, we can define patterns for paths that lead to landing categories and then use the one landing level that works for each particular voyage based on available information. There is no reason we cannot have upper paths organized by departure location, arrival location, and time (specific enough to separate different sailings of the same ship and perhaps even specific departure and arrival dates). If some of the information isn't available for a particular landing category voyage, then that parent path just isn't used for that voyage. That way, there would be some patterns in the upper level paths but still enough flexibility to handle the variety of information available and the variety of uses to which it is put.
Trying to write guidelines for such flexible structures is difficult. They either become too vague for many users to understand or they wind up defining so many different patterns that many users get lost in the complexity. We ran into this problem with the Religious Categories. But we need to figure out how to describe such flexible patterns because we have many topics in our category tree that don't fit a single pattern for the whole world and all time. We need to be able to describe in understandable ways patterns with enough variables and flexibility to accommodate the differences that occur around the world and through different centuries.
So perhaps one of the things that needs discussing is how to describe and communicate a category structure pattern with multiple variables that both sketches out a general pattern and perhaps the developed detail for one area and time while still giving guidance but leaving room for variation as other areas and times are developed later.