What does 'spalla camicia' on a suit really mean?

As soft Neapolitan jackets have become popular, the Italian phrase 'spalla camicia' has often been used to describe their style.
But if it means anything consistently, it actually refers to a narrow point - a way of connecting the sleeve and shoulder of a jacket.
British author Simon Crompton explains this point, as well as the other aspects of a Neapolitan-style jacket that are often associated with it.
Part of a series on the Permanent Style channel, and on the popular website PermanentStyle.com