Can someone explain that rdisk= with boot-uuid= for an dual boot LEO / SNOW

for example:
disk0s1 is Leo , active (standard boot disk), uuid= AAAAA
disk0s2 is Snow Leo, not active (F8>Snow), uuid= BBBBB

What i cant understand is why both are needed, or is rdisk= more urgent for that dual boot problem ?
If from LEO partition is bootet (LEO is active) , does Chameleon loads all of that stuff(/EXTRA) BEFORE i set by F8 booting Snow Leo from other disk ?!

Thanks
I made that dual boot by having noting special (snow leo/leo) in the /EXTRA/Extensions , also no .mkext. All special kext (Leo vs SL) i put the their own System/Extensions