Skulls On Different Pieces of Armour?

TLDR: See photo, what do the symbols mean? Who would wear this loincloth? What do skulls on different parts of the armour signify?

I've been trying to learn the lore and the significance of different markings on space marine armours, specifically pertaining to the Codex Astartes, but I'm struggling to grasp certain things because not all of them seem to be university used or accepted.

I've attached a photo of a loincloth that I don't understand. Is this only worn by command?

The right pauldron is supposed to denote rank I believe, so a skull there is supposed to show that a marine is in a command position? But there are skulls on other pieces of the armour, like on the helmet, the backpack, forearms, knees, loincloths, etc...

Helmet skulls don't always denote rank, I don't think? They can be a display of valor or other importance?

So I guess my question is, are the skulls only worn on different pieces of armour by marines that are in command positions (similar to the right pauldron)? Or can skulls be worn by any marine that isn't in command, granted its NOT placed on the right pauldron? If these skulls don't denote a rank, what are they denoting?

Not to mention the laurels, and other fancy symbols...I'll figure those out later, lol.

Thanks in advance!