cashflow_category.id and cashflow_subcategory.id on a transaction (see Transactions and statements), so they reconcile against an external chart of accounts.
Tools
Assigning to a leaf
A transaction is categorized against a single node in the tree. Once a category has subcategories, assignment must target a leaf subcategory — assigning the parent is rejected. Resolve the rightcategory_id with list_cash_flow_categories, then pass it (the subcategory id, when one exists) to modify_transaction_cash_flow_category with the assign branch.
clear is its own branch rather than an assign with an empty category_id, so removing a category is always explicit and a tagging call can never silently wipe one.
Try it
What cash flow categories do I have set up? Create an outflow category “Software” with a “Cloud hosting” subcategory under it. Tag my last three AWS transactions as Cloud hosting. Clear the cash flow category on that miscategorized transfer.