diff --git a/Score-Calculators.md b/Score-Calculators.md
index b6374ce..40c1ad5 100644
--- a/Score-Calculators.md
+++ b/Score-Calculators.md
@@ -17,6 +17,17 @@ testWeights:
     "Test 03": 100
 ```
 
-The keys are test names (must correspond with the `name` column of the `exercise_test`) and the values has to be integers.
+The keys are test names (must correspond with the `name` column of the `exercise_test`) and the values has to be integers. When all weights are equal, the weighted calculator behaves exactly as uniform calculator.
 
 ## Universal
+
+The most complex calculator which was added last. It holds an AST (Abstract Syntax Tree) of an expression used to compute the score in the configuration.
+
+The AST is encoded directly into Yaml structure -- each node is represented as a collection (object). A node must always have property `type`, which holds a string identifier of the node type (types of nodes are below).
+
+Leaf nodes (without children):
+* `value` - a literal value, which holds one float (the value itself is stored in `value` property)
+* `test-result` - reference to a test score, which is also a float between 0 and 1 (name of the test is in the `test` property)
+
+Internal nodes (functions):
+* `sum`
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