towncrier has a few standard types of news fragments, signified by the file extension. These are:

.feature: Signifying a new feature.
.bugfix: Signifying a bug fix.
.doc: Signifying a documentation improvement.
.removal: Signifying a deprecation or removal of public API.
.misc: A ticket has been closed, but it is not of interest to users.

The start of the filename is the ticket number, and the content is what will end up in the news file. For example, if ticket #850 is about adding a new widget, the filename would be newsfragments/850.feature and the content would be

myproject.widget has been added.