diff --git a/Sensor-API.md b/Sensor-API.md
index f6ede560..57ad77b2 100644
--- a/Sensor-API.md
+++ b/Sensor-API.md
@@ -149,12 +149,12 @@ struct XDRVMAILBOX {
   uint16_t      valid;      // ???
   uint16_t      index;      // Sensor index
   uint16_t      data_len;   // Length of command string
-  uint16_t      payload16;  // ???
-  int16_t       payload;    // ???
+  uint16_t      payload16;  // 16 bit unsigned int of payload if it could be converted, otherwise 0
+  int16_t       payload;    // 16 bit signed int of payload if it could be converted, otherwise 0
   uint8_t       grpflg;     // ???
   uint8_t       notused;    // ???
   char         *topic;      // Command topic
-  char         *data;       // Command string/value
+  char         *data;       // Command string/value - length of which is defined by data_len
 } XdrvMailbox;
 ```