I have been trying to do some reading about catus juice and alumilite and am confused. What is the difference and when is the appropriate useage for each

Cactus Juice is a stabilizing resin.  It is used to fortify the wood itself by impregnating the cellular structure.  This makes the wood harder and more resistant to moisture.  It will not fill voids.  Alumilite is a casting resin that is used to fill voids and cracks in the wood but does not fortify the wood.  They work well together by stabilizing the wood first, then casting with Alumilite to fill in the voids and crack if needed.