A constraint selector specifies an XPath expression that selects a set of elements for a constraint.
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Open the property sheet of a constraint, and enter an XPath expression in the Selector (XPath) field (see XPath abbreviated syntax).
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Click Apply to make available the Properties tool to the right of the field.
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Click the Properties tool to open the selector property sheet.

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[optional] Enter additional properties for the selector, and then click OK to return to the constraint.