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It all depends on how far you want to go down the rabbit hole. XDA has all the files and you can make your own image if clever enough (I'm not that clever anymore). You can also just download an image file already made, but these might not come in the card size you have (I think most are 2 or 8 Gb). Think partitions. You can partition a 1Tb hard drive to be 200Gb and waste the rest. That's what happens with these images. So if there's only an 8Gb image, the rest of your card is ignored.
HOWEVER to finally answer your question; whatever is not used in the image by OS is yours to use as you wish (just not maybe all of your card). Good luck!
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