Optional is used to specify "optional arguments" in functions represented by patterns. The pattern object p gives the form the argument should have, if it is present. The expression v gives the "default value" to use if the argument is absent.
The form s_:v is equivalent to Optional[s_, v]. This form is also equivalent to s:_:v. There is no syntactic ambiguity since s must be a symbol in this case.
The special form s_. is equivalent to Optional[s_] and can be used to represent function arguments which, if omitted, should be replaced by default values globally specified for the functions in which they occur.
Values for Default[f, ...] specify default values to be used when _. appears as an argument of f. Any assignments for Default[f, ...] must be made before_. first appears as an argument of f.
Optional[s_h] represents a function which can be omitted, but which, if present, must have head h. There is no simpler syntactic form for this case.