From my understanding, a cadet branch is when a junior line of a house gains power independently - for example, the junior son being granted a title in his or own right.
This explains why Charles de Valois is considered the head of the Valois cadet branch, insofar as he was created Count of Valois by his father.
That said, when his uncle and cousins died, Philip de Valois, son of Charles, became King of France. At that point, one could consider the Valois to be the senior line in the Capetian house. So why are they considered Valois primarily, and Capetians second?
Is this because Joan, daughter of one of the aforemented cousins and previous Kings, became Queen of Navarre, and was thus considered to still be the senior line of Capetians until her death? If yes, what happens after she died - surely then the senior line had become extinct and the Valois were now entirely the senior branch of the Capetians.
It's all so very confusing. Cadet branches don't make sense to me - who decides these things?
who decides these things?
I don't really think there is anyone who decides them, so much as habit and terminology develop which are followed for consistency. Certainly in the 14th century there was no agreed-upon set of rules for this sort of thing (or the 100 years war wouldn't have happened). The best example is, I suppose, how the changes in dynasty are handled: modern historians see a shift between the Capetians and the Valois kings of France, but it's a change of lineage much less dramatic than, for example, that between Yorkist and Tudor kings or Tudor and Stuart houses in England. It's just that France had a remarkably stable monarchy, with virile kings, so historians are forced to divide the monarchy up somehow.
In terms of how I see it, the House of Valois was a cadet branch of the Capetians up until the death of Charles IV. After his death, the Valois became the senior branch, despite the fact that the new king (Philip VI) was from a cadet branch. From 1322, the Valois were undoubtedly the senior Capetian branch. Like I said, the division between Capetian and Valois (and later Bourbon) branches is really just for the sake of historians - remember that Louis XVI was tried under his 'civilian' name as Louis Capet.
I hope that's made things a bit clearer, but if not please tell me and I'll try to clarify!