# DMN hit policies

The hit policy is a single letter in the corner of a decision table, and it decides everything about how the table behaves when more than one rule matches.

It is the part most often ignored and most often the cause of a surprise in production.

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## When it matters

* Any table where two rules could match the same input.
* Decisions that will be executed rather than only read.

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## The symbols

| **Symbol**       | **Means**                                                         |
| ---------------- | ----------------------------------------------------------------- |
| U (Unique)       | Exactly one rule may match. Overlap is an error, which is useful. |
| A (Any)          | Several may match, but they must produce the same output.         |
| F (First)        | Rules are ordered; the first match wins.                          |
| P (Priority)     | The highest-priority output value wins, regardless of row order.  |
| C (Collect)      | Every match contributes; optionally sum, min, max or count.       |
| R (Rule order)   | Every match, in rule order.                                       |
| O (Output order) | Every match, in output priority order.                            |

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## A few things that catch people out

* Unique is the safest default. It turns overlapping rules into an error rather than a silent choice.
* First hides overlaps behind row order, which makes reordering rows a behaviour change.
* Collect with an aggregator (sum, count) returns one value; without one it returns a list.

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## Related

[FEEL expressionsThe little language inside the cells.](https://help.gocapable.com/diagrams/feel-expressions.html)

[DMN decision table structureInputs, outputs, rules, and where each goes.](https://help.gocapable.com/diagrams/dmn-decision-table-structure.html)

[DMN notation referenceBack to the full DMN reference.](https://help.gocapable.com/diagrams/dmn-notation-reference.html)

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_Pick the hit policy on purpose._