Whats the diff between i/d feline gastrointestinal health & a/d feline critical care canned food?

my cat recently been diagnosed wif colon cancer and the m's seems to be partially blockg her thus she poop wif discomfort. vet prescribed these two canned food, say it will give her the nutrition n not producing too much stool, but i wld like to know more abt the difference, he gave me three cans of each, saying its okay to feed either, are there anything i shd take note while feeding her these? i have already started her the i/d, small amounts everytime, afterwhich shd i try the a/d? or just contnd the a/d? txs for any advice