How to read index of variations in a book?

Next to the end of opening books, there are often index of variations. But how to interpret I am confused. For example, from the ffollowing pic, what does that '13' mentioned after a) 5.Nb3 under 'other moves' mean?

Its not page number, I have checked.

Thanks in advance

It is either page number or possibly game number if the book has complete games.

Keep in mind, page number can be anywhere on the page.  It could be a side note one third of the way down the second column.  Page number does not mean top of the page.  Could even be the last line if test on the page that continues on to page 14.

Again, the only other possibility is a few books index game number rather than page number if the book is complete games.

Can’t be a game number as different openings reference the same number.

Edit: actually I supposed it could be variations discussed in that game text.

I guess it is a pdf book : then the pages can be different...

But ThrillerFan gave the answer : almost every time it is page numbers, sometimes games number (rare).

Here game numbers doesn't make any sense if u think a bit.

Actually it might be game number.  I see no numbers higher than low 100s.  Multiple sidelines could be covered in the notes to the same game.

This must be that green batsford book on the accelerated Dragon as opposed to the light blue one without complete games.  Both are from the late 90s.

And if it is the green book, I recall there being an error in the main game of it was either game 58, 59, 68, or 69, but don't recall which, and I remember it being very difficult to reconstruct the game.

Thanks for thinking and making comment.

Thank you !

Thank you !