What's the difference between instance method reference types in Java 8?

So Java 8 introduces method references and the docs describe the four types.

My question is what's the difference between the two instance types?

1. Reference to an instance method of a particular object.
2. Reference to an instance method of an arbitrary object of a particular type.

Both refer to references but what's significantly different? Is it that the type inference used to resolve them is different? Is it significant that (in their examples) one is a closure and the other is a lambda? Is it something to do with the number of arguments on a method?

1. myString::charAt would take an int and return a char, and might be used for any lambda that works that way. It translates, essentially, to index -> myString.charAt(index).

2. String::length would take a String and return an int. It translates, essentially, to string -> string.length().

3. String::charAt would translate to (string, index) -> string.charAt(index).

With this they mean that you have the following:

1) Can be for example this::someFunction;, this will return the someFunction reference of the current object.

2) Can be for example String::toUpperCase, this will return the toUpperCase method of String in general.

I am not sure if there is an actual difference in behaviour, I think it is just like you can also call static methods on instance variables.