Blank sequences work slightly differently depending on whether or not the head of the expression in which they appear is a symbol with the attribute Flat.
Consider matching the pattern against the expression . If f is a symbol with attribute Flat, then the will be taken to stand for the expression . If f is not a symbol with attribute Flat, then will be taken to stand for the sequence of expressions , .... With a named pattern, such as , x can be used only as an element in an expression. The sequence of expressions , ... is "spliced in" to replace x, thereby usually increasing the length of the expression.
If matches a sequence of length more than one, then the sequence will be represented by a Sequence object. In most uses of , however, the Sequence object will automatically be spliced into another expression, and will never appear explicitly.