DfromLine command converts line oriented text file into a D-file. See the manual DfromLine for the detail.

The command argument field-format-list:

"chapter:+7,word:*"

provides the field name and specifies how to read the field.
In this case

1. after skipping 7 characters, read the field "chapter" to the default delimiter (TAB).
2. then repeatingly read the field "word" delimited by TAB upto the end of line.

Detail of the field-format-list is describe in the manual D_fmt.