I have a songbook where every song has a number, inserted using a counter field (a number field named “Song” and having “Song+1” as value). The book also has page numbers, but the song numbers are more important for the reader. How can I show these numbers instead of the page number in my table of content?
Since your song are numbered with a number range (which is automatically incremented), this song number has no real significance beyond numbering.
You didn’t describe the structure of your book: no chapter at all or several chapters?
##No or single chapter
Style the song title paragraph with Heading 1 or Heading 2 and enable chapter numbering. The TOC will automatically include song number and title
Several chapters
###Songs numbered by chapter
Use Heading 2 and make sure chapter numbering includes both level-1 and level-2 to make the number unique
###Songs numbered continuously throughout the book
Create a list counter style (fifth icon in the side list pane). Call it Song and define its properties in Position and Customize tabs. Alternatively, you can use built-in Numbering 123 (no need to create or modify it).
Next, customise a “list” paragraph style like Numbering 1.
- Modify this style definition 
Outline & Numbering. In the Numbering style menu, selectSongorNumbering 123according to your previous choice. - Set Outline level to the desired level in the TOC (probably at least 
Level 2because you also have chapters here). 
EDIT Removed an ambiguity in style category designation
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Thanks! That sounds like a good approach I will try this asap, what I need is the combination “Several chapters” and “Songs numbered continuously throughout the book”.