What do the codes mean in Box 13 of the estate Schedule K-1?

Credits and Credit Recapture
A: Credit for estimated taxes
B: Credit for backup withholding
C: Low-income housing credit *
D: Advanced manufacturing production credit*
F: Work opportunity credit
G: Credit for small employer health insurance premiums
H: Biofuel producer credit
I
J: Renewable electricity production credit
K: Empowerment zone employment credit
M: Orphan drug credit
N: Credit for employer-provided childcare facilities and services
O: Biodiesel and renewable diesel fuels credit
P: Credit to holders of tax credit bonds *
Q: Credit for employer differential wage payments
R: Recapture of credits
ZZ: Other credits*

* Not supported

See the Schedule K-1 (Form 1041) instructions  for more info about these codes.