danielcweeks commented on code in PR #17822:
URL: https://github.com/apache/iceberg/pull/17822#discussion_r4108044320


##########
format/spec.md:
##########
@@ -654,6 +655,129 @@ Sorting floating-point numbers should produce the 
following behavior: `-NaN` < `
 
 A data or delete file is associated with a sort order by the sort order's id 
within [a manifest](#manifests). Therefore, the table must declare all the sort 
orders for lookup. A table could also be configured with a default sort order 
id, indicating how the new data should be sorted by default. Writers should use 
this default sort order to sort the data on write, but are not required to if 
the default order is prohibitively expensive, as it would be for streaming 
writes.
 
+### Constraints
+
+A **constraint** declares a property that a table's rows are expected to 
satisfy. A constraint's definition is stored in table metadata. Whether a 
constraint holds is recorded for each snapshot, see [Constraint 
Validation](#constraint-validation).
+
+Iceberg does not evaluate constraints. Enforcement and validation are 
performed by engines that write to a table. Iceberg stores constraint 
definitions and records the status that a writer reports for a commit without 
verifying it.
+
+Constraints are added in v4 and are not supported in v3 or earlier.

Review Comment:
   I think this statement should go first.  It's a little odd to interject this 
between the description and definition.



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