You can also call it like an ordinary function to determine if an object is a string.
Comments:
When invoked as a function, returns 1 if x is a string otherwise returns 0.
A type check error occurs if a variable declared as a string is assigned to a dword-sequence.
Supplemental:
The following are also defined in builtins/ptypes.e (not an autoinclude, but used by both pGUI? and mpfr, so either of those will include it):
global type nullable_string(object o) - o must be a string or NULL.
global type atom_string(object o) - o must be a string or an unsigned non-fractional machine-word-sized atom, such as the result from allocate_string?() or a char* from c_func?(), etc.
There are generic versions of the string function implemented as :%opStr in builtins\VM\pType.e and :%opStr0 in builtins\VM\pJcc.e that the compiler only invokes as a last resort, preferring instead to inline that functionality if the argument is known to be assigned or the result variable, if any, is known to be an integer, ie does not require decref/dealloc. The compiler may also completely omit the test and/or any related code, if type inference/propagation determines that the result will always be true or always false.
As already mentioned, builtins/ptypes.e is where the supplemental types are defined.
The sq_str() routine can be used to apply a string() test to all top-level elements of a sequence, eg sq_string({1,1.5,"string",{1,1.5,"string",{}}) ==>; {false,false,true,false}. That routine returns a single true/false if passed an atom or string, otherwise a "flat" sequence of true/false of the same length as the input argument. Note there are no known valid uses of the sq_str() routine, as yet.
