Hello,
The available options are detailed on the page (https://www.phpdocx.com/api-documentation/docxutilities/merge-Word-documents-with-PHP). This page includes a parameters table with all available options, their types, and the documentation: mergeType, enforceSectionPageBreak, numbering, lineBreaks, forceLatestStyles, renameStyles, and importEmbeddedFonts.
The same information is also detailed in the practical guide (https://www.phpdocx.com/documentation/practical/merging Merging DOCX documents section). And the phpdoc documentation included in the class.
For example, to set mergeType as 1:
$merge->mergeDocx('document1.docx', array('document2.docx', 'document3.docx'), 'output.docx', array('mergeType' => 1));
Regards.