Pick what it is. The form then asks only for what that kind needs — and nothing it does not.
A book
One whole book, by its authors or its editors.
A chapter
One chapter inside a book. It needs the book above it.
A book in a series
A volume in a named, ISSN-carrying series.
A book in a set
A volume of a multi-volume set.
Conference proceedings
One volume of proceedings, for one event.
Proceedings in a series
A proceedings volume belonging to a named series.
A conference paper
One paper inside a proceedings volume.
Built, then read back
The Crossref file is built, deposited, and read back afterwards — you see the DOI resolve before anyone is told it is done.