and
A $B$-parameterized family of Dirac operators on $d$-dimensional space gives rise to their determinant line bundle on $B$. If $d = 8k+2$ for $k \in \mathbb{N}$ this has a canonical square root line bundle, the Pfaffian line bundle.
In the path integral quantization of quantum field theory, the partial path integral of an action functional over just the fermionic fields yields in general not a function on the remaining space of bosonic fields, but a section of a line bundle on this bosonic configuration space, the determinant line bundle of the family of Dirac operators. The Lorentzian metric? assumed in relativistic quantum field theory leads in the Wick-rotated theory to the passage to the square root of this line bundle.
Therefore the nontriviality of the Pfaffian line bundle is in these dimensions the fermionic quantum anomaly.
Pfaffian line bundle
The following table lists classes of examples of square roots of line bundles
The general notion of Pfaffian line bundle is described in section 3 of
The string worldsheet Green-Schwarz mechanism which trivializes the worldsheet Pfaffian line bundle, and its relation to string structures that goes bak to Killingback and Edward Witten has been formalized in