diff --git a/enzyme/Enzyme/CApi.cpp b/enzyme/Enzyme/CApi.cpp
index ca71867462f..d6d7b1c6628 100644
--- a/enzyme/Enzyme/CApi.cpp
+++ b/enzyme/Enzyme/CApi.cpp
@@ -340,6 +340,21 @@ void EnzymeRegisterAllocationHandler(char *Name, CustomShadowAlloc AHandle,
     };
 }
 
+
+/// This is the main entry point to register a custom derivative for language frontends.
+/// It should be prefered over trying to register custom-derivatives in the llvm-ir module.
+/// The main reason is that these rules can handle non-default activity cases, e.g.
+/// a function call where a pointer or a float scalar is marked as const.
+/// To get a better low-level understanding, the code in AdjointGenerator.h can be read.
+///
+/// This Function will only handle using ReverseMode AD (either split or combined).
+/// As a high-level example, assume we want to register a custom derivative for a vector resize function.
+/// We pass the mangled name as first argument.
+/// resizing is a simple enough to be handled in the fwd pass, so we just pass a nullptr as RevHandle.
+/// For the forward pass, we will need to resize the shadow of the input (if duplicated), so the CI
+/// is a CallInst of resize on the shadow argument. The IRBuilder B, the shadow argument, and gutils
+/// should all be provided available in the frontend. The last three arguments ...
+///
 void EnzymeRegisterCallHandler(char *Name,
                                CustomAugmentedFunctionForward FwdHandle,
                                CustomFunctionReverse RevHandle) {
@@ -363,6 +378,13 @@ void EnzymeRegisterCallHandler(char *Name,
   };
 }
 
+/// This is the main entry point to register a custom derivative for language frontends.
+/// It should be prefered over trying to register custom-derivatives in the llvm-ir module.
+/// The main reason is that these rules can handle non-default activity cases, e.g. 
+/// a function call where a pointer or a float scalar is marked as const.
+/// To get a better low-level understanding, the code in AdjointGenerator.h can be read.
+///
+/// This Function will only handle using ForwardMode AD.
 void EnzymeRegisterFwdCallHandler(char *Name, CustomFunctionForward FwdHandle) {
   auto &pair = customFwdCallHandlers[Name];
   pair = [=](IRBuilder<> &B, CallInst *CI, GradientUtils &gutils,