I just put what chins are in the cage and the cage number. That way they don't completely destroy their records if they get to the card in the holder. I file the records for each chin under the cage number that the chin is in and keep the file box in a safe place. I will put other information on the cage card if it helps me to remember to give someone meds or when babies were born or need to be weaned. Also, I like to put on the rescue cage cards when they got here so I know at a glance.
I have seen people put information on when the chins should be expected to have babies, how many babies are in the cage, when a certain chin was placed in breeding with another chin, other miscellaneous dates for other things and information on if a chin is available for sale or is sick and needs meds, etc etc.
You can put what makes it easier on you to manage your herd and remember everything.