The beginning of doxygenation:

As a first step, added an @file description for each file.

bacnet-stack/demo/handler/s_upt.c

@@ -43,6 +43,8 @@
 #include "handlers.h"
 #include "txbuf.h"
 
+/** @file s_upt.c  Send an Unconfirmed Private Transfer request. */
+
 void Send_UnconfirmedPrivateTransfer(
     BACNET_ADDRESS * dest,
     BACNET_PRIVATE_TRANSFER_DATA * private_data)