A named way of organizing — the lattice's own unit: one lens that groups concepts into a view.
Greek skhema (form, shape), the same root as schema; scheme kept the sense of an organising plan while schema kept the sense of a declared structure, two English words grown from one Greek shape-word.
One chosen lens laid over a body of concepts, not the only lens available for the same material — which grouping shows up depends on what the scheme is for, and the choosing carries information of its own.