hook_post_update_NAME is similar to hook_update_N but runs after database updates complete. It is intended for updates that rely on the updated schema, entity definitions, or configuration.
The name uses a descriptive identifier rather than a sequential number. This makes post update hooks easier to read and reason about, especially in teams where multiple people might write updates simultaneously.
Use post update hooks when you need to manipulate entities or configuration rather than the raw database. For example, updating content after a new field has been added is a good fit. Post update hooks were introduced because hook_update_N runs before some parts of Drupal are fully initialised, which caused problems for complex updates. Today, post update hooks are the preferred choice for most update tasks that do not involve raw schema changes.