Upper Saxony
It was particularly to distinguish the lands from 'Lower Saxony', a concept which arose later in popular usage (though never enforced) as a term for the original Saxon lands in north and west Germany (where Low German dialects had spread), in what is now the state of Lower Saxony, as well as the adjacent Westphalian region, Holstein and the western part of today's Saxony-Anhalt north of the Harz range (Eastphalia).
Limes Saxoniae
The Limes Saxoniae (Latin for "Limit of Saxony"), also known as the Limes Saxonicus or Sachsenwall ("Saxon Dyke"), was an unfortified limes or border between the Saxons and the Slavic Obotrites, established about 810 in present-day Schleswig-Holstein. 
After Charlemagne had removed Saxons from some of their lands and given it to the Obotrites (who were allies of Charlemagne), he finally managed to conquer the Saxons in the Saxon Wars. In 811 he signed the Treaty of Heiligen with the neighbouring Danes and may at the same time have reached a border agreement with the Polabian Slavs in the east. This border should not be thought of as a fortified line, however, but rather a defined line running through the middle of the border zone, an area of bog and thick forest that was difficult to pass through. According to Adam of Bremen's description in the "Gesta Hammaburgensis ecclesiae pontificum" about 1075, it ran from the Elbe river near Boizenburg northwards along the Bille river to the mouth of the Schwentine at the Kiel Fjord and the Baltic Sea.