It seems I could have placed:

- Dodgy Right Knee as either a Physical Feature or a Physical Trauma

- Social Anxiety either as a Social Feature or a Social Trauma

What's the difference?

My interpretation is that if the players decide on it prior to the start of the game, it is a negative feature. If it develops due to the acquisition of stress, it is a trauma.

Thanks Chryondave

Traumas heal over time, whereas features are (generally) something permanent. So, if the dodgy right knee is the result of a recent injury and is expected to heal, you would list it as a trauma when making your character, and would remove it during play after enough time passes (assuming you survive long enough!). If it's something you've had for years and don't expect to go away anytime soon, it could be a feature. In any case, you can absolutely start the game with a trauma, if you choose (page 23).

That's my take, anyway!