harry james:
THis has nothing to do with a split rest you are right this must be a minimum of 3 hours and cant be taken in blocks of 1 hour. I think it makes it easier for them to see your breaks have been taken.
It has everything to do with split daily rest. 
Because the digi tacho still works to the pre April 2007 EU regulations the asterix denotes any break of over 1 hour which you used to be able to use toward a split daily rest.
Pre April 2007 you could split the rest into 3 parts, with the shortest being 1 hour and the longest and last part being at least 8 hours, totalling 12 hours, which is why it marks any break/rest of at least 60 minutes.
The asterix now has no meaning unless it is marking a period of at least 3 hours.