Each preset in Xpiks can be a member of a “group”. Preset group is just a convenience feature to categorize presets in the context menu “Insert preset”. It’s not uncommon to have many presets in Xpiks and it’s quite a mess to have them all listed in the menu.
That’s how preset groups work: if a preset is categorized to a group, the menu will show a group with this preset. If many presets are categorized to a single group, it will have all of them in the menu. Presets which are not categorized are shown below all groups.
You can categorize preset to the group in the Presets dialog where you can either choose existing group or create one.