diff --git a/taxcalc/validation/taxsim35/Differences_Explained.md b/taxcalc/validation/taxsim35/Differences_Explained.md
index 0a9282461..c43ad1777 100644
--- a/taxcalc/validation/taxsim35/Differences_Explained.md
+++ b/taxcalc/validation/taxsim35/Differences_Explained.md
@@ -17,4 +17,5 @@ This document explains the sources of known differences (that exceed $1) between
 * Numerous records in the test files with differences in the recorvery rebate credit amount (RRC). The reasons TAXSIM-35 shows different results vary and include: TAXSIM-35 not counting qualifying children (e.g., file "a", id 7);  TAXSIM-35 not differentiating single/head of household filing status (e.g., file "a",id 31); and TAXSIM-35 not counting Economic Impat Payment 2 (e.g., file "a",id 33); TAXSIM-35 counts wrong number of child (e.g., file "a",id 59). Note that some of these are not errors per se, but can be related to different variable inputs in the two models.
 
 
-## 2021
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+## 2021
+* Child tax credit is categorized as nonrefundable child tax credit. The variable `c11070`, the refunded Child tax credit, is counted as zero in Tax-Calculator. TAXSIM-35 calculates the amount of Child tax credit as non-zero (it does not differentiate refundable vs nonrefundable credit).
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