I previously posted on why the making of individual kelim such as the shulchan, menorah, etc, are not counted as separate mitzvos. The Ramban writes that making these items are just hechsheiri mitzvah; e.g. making the menorah is not an independent mitzvah, but is just a necessary ingredient to fulfilling the mitzvah of lighting menorah lamps; making a mizbayach is not an independent mitzvah, but is just a necessary ingredient to performing the mitzvah of offering korbanos. The Rambam opined that the making of kelim is not counted as independent mitzvos because these are all just parts of the larger mitzvah of making a Mishkan.
Is making and wearing bigdei kehunah a mitzvah? The Rambam (aseh #33) counts it as one, but the BH”G disagrees. Ramban explain l’shitaso that the wearing of bigdei kehunah is not itself a mitzvah, but is just a hechsher mitzvah to performing avodah. BH”G only counted doing avodah without bigdei kehunah as an issur. The Megillas Esther defends the Rambam l’shitaso as distinguishing between a hechsher mitzvah and a cheilek of a mitzvah. There are many hechsheiri mitzvah which do count as independent mitzvos, e.g. shechting the korban pesach, preparing the ashes of parah adumah – and, according to the Rambam, bigdei kehunah. What does not count as an independent mitzvah is when something is just a part of the broader mitzvah, a cheilek of the mitzvah, e.g. making kelim for the mikdash. It is easy to impose this distinction on mitzvos after the fact based on what the Rambam did or did not count, but I am unclear as to what exactly defines the difference between the two categories. Why indeed is shechting the korban pesach an independent mitzvah and not just a cheilek of the larger mitzvah of properly offering the korban pesach? What rule does one use to determine hechsher or cheilek?