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Earlier sup720s had bootflash (64KB?), which was too small to contain later IOS images. So, Cisco provided a upgrade kit that provided a larger "bootdisk" that took the place of bootflash, they also map the latter (name) into the former. (Also, newer sup720s come with bootdisk too.)
Besides bootflash and bootdisk being determined by which is physically on the sup card, I believe processor boot ROM version images come into play.
Your other question about SP vs. RP boots, is also a bit of a muddle. Original setups (old hybrid mode) has SP boot for L2 and RP boot for L3, now the normal (native mode) manages both, and appears to hide (or rename, or make inaccessible?) some SP flash when IOS is controlling device. So, when you access the two different processors, there can be differences in what they "see". I.e. there's a bootflash for both, but the newer native mode IOS images don't use SP bootflash, I believe.
