Drupal Stacks Module naming convention is inspired by the drupal naming convention.

Drupal Stacks module naming convention

[machinename]--default.html.twig
[machinename]--[variation].html.twig
page--stacks.html.twig
page--stacks--[group].html.twig
page--stacks--[machinename].html.twig

Preview image naming conventions

Stacks allows site-builders to add preview images for each template/theme variation.

default.jpg
default--[variation].jpg
[machinename].jpg
[machinename]--[variations].jpg

More about the Drupal Stacks module.

Thanks, Lehel