The requirements for the searchCondition in a constraint are stricter than the requirements for the searchCondition in a WHEN clause. Only the dynamic parameters restriction in that list apply to the WHEN clause. (Note to self: We should add tests to verify that those restrictions don't apply.)
Actually, I think the current placement of the searchCondition topic (under CONSTRAINT clause) is suboptimal. It would be better to have it in the SQL expressions section, and link to it from CONSTRAINT clause, WHERE clause, HAVING clause, CASE expression, and from the ON clause specifications in the INNER JOIN and LEFT/RIGHT OUTER JOIN topics. The searchCondition topic could again link to booleanExpression.
In the SQL standard, the production rule for searchCondition is as simple as <search condition> ::= <boolean value expression>. So one possible short-cut we could take (one which the topics for WHERE, ON and CASE already take) is to link the WHEN clause specification directly to booleanExpression. (This still leaves the HAVING clause with a reference to searchCondition even though it doesn't have the restrictions of the CONSTRAINT clause, so in that case we might want to update the HAVING clause with a reference to booleanExpression too.)
Essentially, the searchCondition in the WHEN clause is the same as in the WHERE clause, except that the restrictions already listed for the triggeredSQLStatement also apply to the WHEN clause.
And by the way... Boolean expressions are defined two different places: here in a paragraph of the top-level SQL expressions topic, and here as an independent sub-topic of SQL expressions. The former links to the latter, but the latter does not link to the former. The latter is what other topics typically link to. Unfortunately, that topic does not mention that a boolean expression might also be a general expression that returns a boolean value. It would be good to have that information in that topic too, so that it doesn't give the impression that a boolean expression has to use one of the boolean operators listed in the table.