The label for a group $G$ has the form $\mathtt{N.i}$, where $\mathtt{N}$ is the order of the group and $\mathtt{i}$ is the small group id that distinguishes groups of the same order (as determined in GAP). If a group is not in GAP Small Groups database, we replace $\mathtt{i}$ with an incrementing letter code, assigning labels to groups as they are added to our database.
Subgroups are labeled using the ambient group, index, and an ordering of subgroups up to automorphism and then up to conjugacy with the same invariants.