The Sequence operator reads all rows from all its inputs, in order, and without modification. It returns all rows it reads from its last input only.
This operator is typically used in execution plans where some branches are needed to do preparation work (e.g. to invoke a Table-valued Function operator) and then other branches produce the final output, or multiple branches need to do work in sequence (such as in wide data modification plans). The operators on the non-last inputs often do not produce any output at all, but even if they do the rows returned are discarded. Only rows returned from the last input are returned to the parent of the Sequence operator.
Note that this is quite difference from sequences of numbers in SQL Server, which act similarly to identity integers.