There is a limit on the amount of money that you can donate each year to Foster Care organizations.
The limit is $500 for a single filer and $1000 for filing jointly.
So in any year that you donated more than $500 as a single or $1000 as joint filer, you will carry over the amount that you could not get credit for in that year.
As an example: In 2015, you filed joint and donated $1400 to foster care. You were able to claim $1000 and had a $400 carryover The original carryover for 2016 is $400
In 2016, you donated $1000 and your carryover in 2016 becomes $400 in the first column and $0 in the second column because you used none of the original credit that year
In 2017, you donated $500, You can claim up to $1000 so your first column with use $400 of the original credit and you deduct $900 total ($500 donation plus $400 carryover)
In 2018, you donated $2000 so you can get credit for $1000 and then put $1000 into the carryover for 2019
In 2020, if you had donated zero, you could still take the 2019 carryover and get the $1000 credit.
