Spin structures and the divisibility of Euler classes

In this short article we give a geometric meaning of the divisibility of $KO$-theoretical Euler classes for given two spin modules. We are motivated by Furuta's 10/8-inequality for a closed spin $4$-manifold. The role of the reducibles is clarified in the monopole equations of Seiberg-Witten theory, as done by Donaldson and Taubes in Yang-Mills theory.