A tag is a short consistent label: entered early, moved stop, oversized, skipped the plan, chased. Applied to every trade that contains an error, the tags turn a pile of regrets into a table.
The payoff is in the counting. Once you have forty or fifty tagged trades you can ask which tag is most frequent and which has the largest total cost in R. These are often not the same. A rare mistake with big losses can matter more than a common one with small ones.
Keep the list short, around six to ten tags, and define each so that two reviews of the same trade would pick the same tag. A list that keeps growing makes counts meaningless because the same mistake ends up under different names.
Tag the behaviour, not the outcome. A trade that lost within the plan has no mistake tag. Tagging losses as mistakes teaches you to see bad luck as error and trains the wrong response.
Reflect does not have a tagging feature of its own. It looks for a fixed set of costly habits in your results, and you can keep your own tag list wherever you keep your journal notes.