Stefano showed me the book - he did a great job at finding it since the scenes across the entire Duomo are many!! - so I can also tell you that the parvis is the pavement area in front of the entrance, once you go up the steps to go in but before you step into the cathedral.
It is called "sagrato" in Italian and there are several scenes across the area in front of the 3 doors. The Duomo has an entry ticket to go in and see the great majority of the pavement, which is a masterpiece! Actually most of the pavement IS VISIBLE year round, it is only the area right in front of the altar that is covered because there are the pews set up and the boards placed under the pew protect the floors. There is a need to protect many of the scenes because before this was done, they had started to be worn down from feet passing over them. Most of the scenes now have cords around the area so that you can admire them, without stepping on them.
In any case, every year there is a period where they remove the pews and actually uncover that area as well, so that's when the entire floor is visible.
As for the scene you have in that photo, the book does not indicate who or what. But from similar scenes across the iconography in artwork here, I can tell you it is not the Adoration of the Magi. You are missing baby Jesus and the three wisemen would be very distinctive, each has to be different in dress and bearing gifts. This actually looks like the main figure is a bishop and the figures in the background are also wearing fancy cloaks so am thinking they are religious figures as well. Considering that these are outside the cathedral itself, they are not among the "top" masterpieces as far as the mosaics go but they are likely still important figures. I am thinking but don't know for sure.... that it could be someone of the Piccolomini family.
Inside the cathedral, the Piccolomini Library is dedicated to Pope Pius II but he was also a cardinal in Siena before he became pope. His nephew later became archbishop of Siena and eventually also pope (Pope Pius III) and commissioned the works in honor of his uncle inside the cathedral.... these scenes might actually refer to either one of them or to Pope Pius II (he's more famous because he's the one that built Pienza, the "ideal city", not far from Siena). This is just a theory, the book we have does not label them other than "mosaics in the parvis area".
I hope this is useful as more info to have on the cathedral in Siena and a piece of its history. Have you checked out some of the photos we have of the cathedral on our page here:
https://www.discovertuscany.com/siena/siena-churches/duomo.html ? I'll see if I can add more to the gallery at the bottom.