Tags
A second, free-form way to group transactions across categories — for a trip, a project, or a renovation.
A transaction has one category. It can have any number of tags.
That is the whole distinction, and it matters: categories are structure and feed the reports; tags are for the cross-cutting questions the category tree cannot answer. What did that holiday cost, across flights and restaurants and the car hire. What has the renovation run to so far, across the builder and the appliances and the trips to the hardware shop.
Using them
Add a tag on a transaction, or to several at once from the transactions view. The tag's own page shows everything carrying it and what it adds up to, which is usually the number you wanted.
Tags do not change any report total. A transaction tagged holiday is still whatever category it was; the tag is an extra lens, not a reclassification.