+ comment on macro eps_direct_vs_python

Tests/Suite/Common/TestSuite.h

@@ -38,6 +38,11 @@
 #define DISTRIBUTION_TEST(test_fixture, name) TEST(test_fixture, name)
 #endif
 
+// The macro eps_direct_vs_python allows us to set different tolerances for
+// - direct simulations (fully in C++),
+// - simulations that are exported from C++ to Python, then executed as Python scripts.
+// Python tests require larger tolerances because parameters are not written with
+// full double precision to exported scripts.
 #ifndef PYTHON_STD_TEST
 #define eps_direct_vs_python(eps_direct, eps_python) eps_direct
 #else