As we know, the particle order from normal box is determined by a sort of the following fields: FoF group number, Subfind subhalo number, binding energy, nearest FoF group number.
So, when we have no FoF and Subfind for subboxes, how is the particle ordered in subbox data?
Because there are no group catalogs run when subboxes are saved, they are not "group ordered" (like the main snapshots).
You should treat the subbox data as unordered, containing all particles/cells within that spatial sub-region. If you want to find objects in these subboxes, the only dataset of relevance is Subbox Subhalo List, which may be useful.
Thank you Dylan for the answer.