There isn't a perfect answer to your question so you need to work out which method you prefer.

Option 1: Do the Heading 6/7 workaround that you already read about, then after the PDF is created, relocate the Bookmark levels for the Appendices. This option gives you a lot more functionality in terms of captions and page number fields which include the appendix levels.

Option 2: Make a new series of styles for appendix headings which are numbered independently of the Headings. Include in the style paragraph format settings that the outline level is '1' for Appendix 1 and '2' for Appendix 2 styles. This option won't have the same functionality with page numbers but won't force you to update the PDF's bookmarks every time you export to PDF.

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Andrew Lockton
Chrysalis Design, Melbourne Australia