Original: Initially the SED had a branch in West Berlin, but in 1962 that branch became a separate party called the Socialist Unity Party of West Berlin (Sozialistische Einheitspartei Westberlins - SEW).
Simplified: At first the SED had a branch in West Berlin, but in 1962 the West Berlin branch was separated from the SED proper and became a "separate" party called the Socialist Unity Party of West Berlin (Sozialistische Einheitspartei Westberlins - SEW).