Two examples of NNDs used to test the significance of OC candidates. The solid black line shows the NND of nearby field stars. The blue line shows the NND of distances between cluster members. For a cluster to be significant and not simply a selection of unclustered field stars, the cluster NND must be incompatible with being drawn from the field distribution. For later illustrative purposes, the NND of a cluster member to the nearest field star is shown by the dashed orange line, although this is not used for the CST. In the upper plot, an OC candidate detected by HDBSCAN and crossmatched to the well-characterised OC NGC 6830 is shown, which has a clearly different NND to field stars with a significance of over 20σ. In the lower plot, a false positive OC detected by HDBSCAN in field 19 is shown that has a significance of 0σ.