Every one is different. I mark my primaries, in litres, by pouring in a kg at a time, and marking it with a Sharpie, to make it easier to guestimate how much to make so there is plenty for topping up, etc. A kg isn't EXACTLY 1 L, but it's sure close enough for me. I mark each carboy with total volume.
I don't mark carboys, as I use them only for aging, and they only have 2 settings: Empty and Full.
Like @KCCam, when I get a new primary (in the 7-10 gallon range), I fill it with a 1 quart measure, marking the outside with a Sharpie at each gallon.
For making kits, it makes more sense to mark the fermenter in liters, although with my wide mouthed fermenter, the markings would be close enough together that I'd mark every 2 liters instead of 1. However, 19 and 23 would be marked.
For grapes and juice, I use larger containers, e.g., 32 gallon brutes for grapes. Mine are grey (couldn't find white locally) and are not translucent, so marking them doesn't work. However, for grapes and juice I know the amount before I start as I'm not diluting anything, so markings don't matter.