diff --git a/substrate/primitives/version/src/lib.rs b/substrate/primitives/version/src/lib.rs
index b3ddb7d7fe..da2cb342d2 100644
--- a/substrate/primitives/version/src/lib.rs
+++ b/substrate/primitives/version/src/lib.rs
@@ -98,6 +98,11 @@ pub mod embed;
 pub use sp_version_proc_macro::runtime_version;
 
 /// The identity of a particular API interface that the runtime might provide.
+///
+/// The id is generated by hashing the name of the runtime api with BLAKE2 using a hash size
+/// of 8 bytes.
+///
+/// The name of the runtime api is the name of the trait when using `decl_runtime_apis!` macro.
 pub type ApiId = [u8; 8];
 
 /// A vector of pairs of `ApiId` and a `u32` for version.