The label/lambel, like other similar signs, indicates a brisura of the main coat of arms. Our coat of arms is a "cross brisata in lambello", it means that whoever has that coat of arms is the son of someone who has the same coat of arms without lambello. Specifically ours is the coat of arms of the Prince of Piedmont, son of the Duke of Savoy who has the same coat of arms without lambello.
Source

In heraldry, a label (occasionally lambel, the French form of the word) is a charge resembling the strap crossing the horse's chest from which pendants are hung.
It is usually a mark of difference, but has sometimes been borne simply as a charge in its own right.
The pendants were originally drawn in a rectangular shape, but in later years have often been drawn as dovetails. The label is almost always placed in the chief. In most cases the horizontal band extends right across the shield, but there are several examples in which the band is truncated.
Source Wikipedia