DMN editor

DMN exists because decision logic written as paragraphs becomes ambiguous the moment there is more than one condition. A decision table cannot be ambiguous: every combination either has a result or is visibly missing one.

#What it is for

Instead of	Model
A paragraph describing eligibility rules	A decision table with one row per rule
Nested gateways in a BPMN process	A single decision task backed by DMN
A spreadsheet nobody can find	A decision that lives with the process it serves

#What the editor gives you

The DMN editor showing a decision requirements diagram in which Decision 2 and Decision 3 feed Decision 1, with a shape palette on the left and a properties panel listing General, Documentation, Varia — Two supporting decisions feed a third, and the panel on the right holds the properties of whichever element is selected.

Decision tables. Rows, hit policies and the completeness check.
Decision requirement diagrams. How decisions depend on each other and on input data.
Standard XML. DMN that other tools can read.

#A few things that catch people out

The hit policy matters. Unique, First and Collect give genuinely different answers to the same table.
A table with gaps is not an error, but it is usually a bug in the rules. The editor shows you the gaps.
DMN pairs with BPMN rather than replacing it: the process says when to decide, the decision says how. See BPMN editor.

BPMN editor

The process around the decision.

Diagram types

When you need DMN at all.

Reference

DMN version and hit policies.

Rules you can check, not prose you have to interpret.