Difference between F[_] and F[T] In Scala when used in type constructors

Question

This question is about _ as used in type constructor and not when used in defining existential types.

So the question is what is the difference when _ is us

Answer 1

The only difference I can think of is that with F[_] the parameter itself can have as many holes as possible...that is F[_] can become F[Int] or F[Future[Option[Int]]] etc...while when you have F[T] the T can only be a proper type...that is F[String] or F[Int] etc.

Is this a correct assumption?

Note that Future[Option[Int]] is a proper type of the same kind as Int or String. We can convince ourself by using :kind command in Scala REPL

scala> :kind -v Future[Option[Int]]
scala.concurrent.Future[Option[Int]]'s kind is A
*
This is a proper type.

scala> :kind -v Int
Int's kind is A
*
This is a proper type.

To drive the point, consider the following complicated-looking type

Function3[Int, Tuple2[Double, List[Int]], Char, Future[Either[String, Int]]]

It is still just a simple concrete * type

scala> :kind -v Function3[Int, Tuple2[Double, List[Int]], Char, Future[Either[String, Int]]]
(Int, (Double, List[Int]), Char) => scala.concurrent.Future[Either[String,Int]]'s kind is A
*
This is a proper type.

Hence we see the shape of Future[Option[Int]] is simply *, and F[_] does not need any extra "holes" to fit it. Both F[_] and F[T] type constructors take a type argument of exactly same shape, namely *, no more, nor less. For example, let us try to fit in more than it can handle

trait Bar[F[_]]   // Bar is type constructor of higher order kind  - shape (* -> *) -> *
def g[F[_]] = println("g takes type constructor type argument of * -> * shape")

scala> g[Bar]
        ^
       error: kinds of the type arguments (Bar) do not conform to the expected kinds of the type parameters (type F).
       Bar's type parameters do not match type F's expected parameters:
       type F has 1 type parameter, but type _ has 0

This errors because actual (* -> *) -> * shape of Bar does not fit expected * -> * shape of F.