Examples of class A (top row), class B (middle row), and class C (bottom row) clusters. The columns represent, from left to right, a distribution of the member stars (in blue) and field stars (grey) for: i) position in (α, δ), ii) proper motions in (μ_α^*, μ_δ), iii) distribution in (ϖ,μ_α^*), and iv) a CMD in G vs. G_BP − G_RP. Rows correspond to OCs UBC 257, UBC 478, and UBC 669, respectively. Classes A, B, and C correspond to different levels of reliability (see Sect. 4.2).