For a custom block, the "minecraft:creative_category" object sets the category the item will appear in in the inventory and creative menu by accepting a "category" tag and a "group" tag.

The "group" tag is not required, as it only serves to put a block into a collapsible category, and those categories do not exist for all blocks.

When leaving out the "group" tag, blocks that have been assigned the category "Construction" appear in the Nature tab, and blocks that have been assigned the category "Nature" appear in the Construction tab.

How to Reproduce:

1. For any given addon that adds custom blocks, tag a block's "minecraft:creative_category" object with "category": "Construction" and "group": "itemGroup.name.stoneBrick".
2. In Minecraft, observe the categorization of the block. It will appear in the "Construction" tab (1) and show at the end of the "Decorative Stone" block group.
3. Remove the entire "group": "itemGroup.name.stoneBrick" portion of the "minecraft:creative_category" object
4. In Minecraft, observe the categorization of the block. It will appear in the "Nature" tab (4) just before the "Ores" block group.
5. The same can be done with "category": "Nature" and, for example, "group": "itemGroup.name.stone". It will either appear in the proper Category and Group, or appear in the incorrect Category altogether.