Cost database: categories
Categories are the NRM (or custom) folders that items sit in. They are a tree, not a flat list.
Open the Categories tab. You see Group → Element → Sub-element folders.
The path on each row is the official address in the tree (for example 03.02). Do not think of it as a parent-only list.
You can add a tenant category. Soft-delete hides it from new work. Old estimates still name the category they used.
Examples
Find brickwork
Switch the framework to NRM2. Walk Superstructure → External walls. Items for facing brick sit under that path.