Do you mean in terms of judging people who break it? It's an interesting question. At zeroth order, solidarity seems purely a pragmatism: we mostly agree X must occur, but for X to occur we must act as one. But then at next order, there is a moral dimension to whether you maintained solidarity or let your peers down. At next order you have the moral quality of organisations that defy free will, e.g. totalitarian regimes that demand solidarity. It's a nicely messy area.