The shortest common supersequence (scs) is a common supersequence of minimal length. In the shortest common supersequence problem, the two sequences X and Y are given and the task is to find a shortest possible common supersequence of these sequences. In general, the scs is not unique.
For two input sequences, an scs can be formed from an lcs easily. For example, if X and Y, the lcs is Z. By inserting the non-lcs symbols while preserving the symbol order, we get the scs: U.
It is quite clear that for two input sequences. However, for three or more input sequences this does not hold. Note also, that the lcs and the scs problems are not dual problems.