Ginger wrote:
To amplify Mosc's point about the quality of the text, the Marine list has the following statement
Quote:
"Space Marines are organised into small formations called detachments. Each detachment is made up of three or more units, and may also include a number of extra units called upgrades."
This tells me that should I feel inclined to, I need only field three tacticals - not that I can imagine many players paying 300 points to do so

OK now I see what is happening here. If you will excuse me, this reading is exemplar of incorrect formal logic (i.e. they require an assumption in order to say what you think they do). What you have to recognise is that just because ALL marine formations must have AT LEAST three units does not mean NO marine formations must have MORE than three formations.
I apologise for the abstract nature, but consider this analogous statement: "All motor vehicles have 3 or more wheels."
You cannot conclude from this sentence that a Mondeo can have exactly three wheels. You don't know how many wheels a Mondeo has until you see another statement that tells you: you only know it has some number more than two. Basically, having three wheels (or units) is a necessary but not sufficient condition for being a motor vehicle (or a legal formation).
This becomes even clearer when you read the next sentence for context, because it actually spells this out in exactly this way:
Quote:
The detachments that may be taken in a Codex Astartes army are shown on the chart that follows. The chart also shows what units comprise the detachment, what upgrades are allowed, and its points cost. For example, an Assault detachment consists of four Space Marine Assault units for 175 points, and may include the Commander and Vindicator upgrades at an additional cost in points.
So all detachments have three or more units, and assault detachments have four units. If an assault formation had either three or four units, it would say so.
Of course, the statement that detachments have at least three units is demonstrably wrong anyway, since some detachments have fewer than three units

Now, I will totally agree the rules are hardly the pinnacle of accurate formal logic, but what I am trying to illustrate is that the rules
are in fact internally consistent, and that you have to read lots them against their logical definitions in order to conclude that leaving units out of a formation s legal. In fact, even if you read all the ambiguous rules so far presented in this way, there is still nothing to tell you what you do with the units you are leaving out - you could just as easily conclude that you can deploy them on the table as leave them off entirely, or give the formation blast markers, etc etc.
Basically, you have to try really hard to make the rules fit this, and once you do there are still rules missing to explain how it can work.