diff --git a/common/constants.cc b/common/constants.cc
index 324bdc5..88d0445 100644
--- a/common/constants.cc
+++ b/common/constants.cc
@@ -86,13 +86,21 @@
 const char kPrefsUrlSwitchCount[] = "url-switch-count";
 const char kPrefsWallClockWaitPeriod[] = "wall-clock-wait-period";
 
+// These four fields are generated by scripts/brillo_update_payload.
 const char kPayloadPropertyFileSize[] = "FILE_SIZE";
 const char kPayloadPropertyFileHash[] = "FILE_HASH";
 const char kPayloadPropertyMetadataSize[] = "METADATA_SIZE";
 const char kPayloadPropertyMetadataHash[] = "METADATA_HASH";
+// The Authorization: HTTP header to be sent when downloading the payload.
 const char kPayloadPropertyAuthorization[] = "AUTHORIZATION";
+// The User-Agent HTTP header to be sent when downloading the payload.
 const char kPayloadPropertyUserAgent[] = "USER_AGENT";
+// Set "POWERWASH=1" to powerwash (factory data reset) the device after
+// applying the update.
 const char kPayloadPropertyPowerwash[] = "POWERWASH";
+// The network id to pass to android_setprocnetwork before downloading.
+// This can be used to zero-rate OTA traffic by sending it over the correct
+// network.
 const char kPayloadPropertyNetworkId[] = "NETWORK_ID";
 
 }  // namespace chromeos_update_engine