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).
Lower Saxony
The name of Saxony derives from that of the Germanic tribe of the Saxons. Before the late medieval period, there was a single Duchy of Saxony. The term "Lower Saxony" was used after the dissolution of the stem duchy in the late 13th century to disambiguate the parts of the former duchy ruled by the House of Welf from the Electorate of Saxony on one hand, and from the Duchy of Westphalia on the other.