There are API endpoints for retrieving the test instances that belong to a particular test set, but there is no information about what order these tests should be in.

In the PractiTest UI it's possible to change the order of tests within a test set and have it saved at the https://prod.practitest.com/p/{{project_id}}/sets/{{set_id}}/sort_instances page, but there is no equivalent operation for getting or setting the order of tests from the API.