So you're just incorrectly substituting the word "rotation" with "filler"? Quote from the blm balance page you linked above says "In order to get enough MP to sustain the standard rotation, two MP ticks are required. For this purpose, at least two spells are needed in Umbral Ice to act as “filler” spells." So why would they need to differentiate between rotation and filler if they were interchangeable? Can't answer your question on PLD as I don't play it but I'm sure someone else will chomp down on that bait and correct you.