Graham,

Not sure this is anywhere near a complete or accurate answer, but when the army dispensed with the great kilt and opted for the philabeg box-pleating was introduced to economise of the amount of material being used. When the pattern was matched, the style was to centre the same line on every pleat ("pleating to the line"). Pleating to the line is very military, thus some pipe bands adopt this style.

The modern style for civilian kilts, however, is for the pattern, or sett, of the tartan to be re-created where the pleats are formed (ie the repeat of each sett is hidden within each pleat "pleating to the sett").

Today the pleats of the kilt can be formed in two ways: a standard continuous pleat, the knife-pleat, or the box-pleat which takes the form of a Z joined by its base line to a mirror-image of itself. Box-pleats are, however, more complex to form and add more to the price of a kilt than is compensated for by any saving in material, in my opinion.

Best,
Daniel