diff --git a/Cargo.toml b/Cargo.toml
index eac1783..40a0dd6 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -38,5 +38,5 @@ thiserror = "2.0"
 vergen-gitcl = { version = "1.0", features = ["build", "cargo", "rustc", "si"] }
 
 [features]
-bin = ["anyhow", "env_logger"]
+bin = ["anyhow", "env_logger", "flatmap"]
 flatmap = ["csv"]
diff --git a/src/main.rs b/src/main.rs
index e37c513..49d2788 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -15,13 +15,16 @@ struct Args {
 
 #[derive(Debug, Subcommand)]
 enum Commands {
+    /// Generate register interface code.
     Generate {
         /// Output directory.
         out: path::PathBuf,
     },
+    /// Generate flattened register map in CSV.
     #[cfg(feature = "flatmap")]
     Flatmap {
         /// Flattened csv out path.
+        /// Print to stdout by default.
         flatmap: Option<path::PathBuf>,
     },
 }