As long as the external-id data type does not exist, we will start with a basic setting like this:

$wgWBRepoSettings['statementSections'] = array(
	'item' => array(
		'statements' => null,
		'identifiers' => array(
			'type' => 'propertySet',
			'propertyIds' => array(
				'P61',
				'…',
			),
		),
	),
);

This is what this setting does:

• It's only active for items. Properties will not have sections in addition to the default "Statements".
• There will be a default "Statements" section for everything that does not fit to an other section.
• There will be an "Identifiers" section for specific property ids. The section heading message is https://translatewiki.net/wiki/MediaWiki:Wikibase-statementsection-identifiers/en.

Later, when the external-id data type finally exists, the setting will be changed:

$wgWBRepoSettings['statementSections'] = array(
	'item' => array(
		'statements' => null,
		'identifiers' => array(
			'type' => 'dataType',
			'dataTypes' => array( 'external-id' ),
		),
	),
);