Mm good question. Some have compared the transitional to Lacan's objet a, something which stands as a unique representation for something lost (but as I argued, 'loss' isn't the right word for Winnicott). So it'll look different for everyone, the importance is how it functions rather than what it is.

So the analyst could certainly work this way, as a space between self and other, inner and outer. Adam Phillips insists that the interpretation (as intervention) can operate in a transitional space, bringing the inner to the outer but never leaving it in one place. There are perhaps many clinical uses for this term!