Book numbering properties window – Section and Sub-section grayed out

In FM 12, I have a book composed of one front-matter file and 9 chapter files. The chapters (1-9) auto-numbered without any problems and pagination is correct. I want to add section and subsection numbers. Within each file I added autonumber format tags to the headers used specifically for section and subsection titles on the first chapter page and autonumber format tags within the body of the chapters. The autonumber tags are –

<$chapnum>.<$sectionnum>\t

<$chapnum>.<$sectionnum>.<$subsectionnum>\t

for paragraph tags Heading 2 and Heading 3, respectfully.

When I select numbering in the book file window and click on either the Section and Sub-section tab, the windows are grayed out. I know I'm missing something obvious. I've gone through help files, manuals and tutorials for hours and can't find what I need. Any advice about where I can look to sort this out?

Thank you.