Dsort command sorts D-records by field values. See manual Dsort for the detail.

This command argument key-field-list:

word:f,chapter:n

specifies the sort keys. In this case,

the first key

"word" field folded in upper case (without case distinction).

the second key

"chapter" field as numeric value.

For the detail of the key-field-list, see the manual Dintro.