-Note that string_base62() returns its data in a static storage block, so it
-must be copied before calling string_base62() again. It always returns exactly
-6 characters.
+The second part is the current pid, and supports 64b [31b] PIDs.
+
+The third part holds sub-second time, plus (when localhost_number is set)
+the host number multiplied by a number large enough to keep it away from
+the time portion. Host numbers are restricted to the range 0-16.
+The time resolution is variously 1, 2 or 4 microseconds [0.5 or 1 ms]
+depending on the use of localhost_nubmer and of case-insensitive filesystems.
+
+After a message has been received, Exim ensures that the timer has ticked at the
+appropriate level before proceeding, to avoid duplication if the pid happened to
+be re-used within the same time period. It seems likely that most messages will
+take at least half a millisecond to be received, so no delay will normally be
+necessary. At least for some time...
+
+Note that string_base62_XX() returns its data in a static storage block, so it
+must be copied before calling string_base62_XXX) again. It always returns exactly
+11 (_64) or 6 (_32) characters.