This is the next step in Clickups intended evolution from task list towards a flexible productivity platform. Item types allows to create names for items types and to create or select custom fields that are used by default for each item type.
In another feature request already the naming of 'item' has been proposed instead of 'task' but the current request is about the ability to create item 'types' that are available on the space level and down, which can hold their own set of custom fields.
This concept is the main principle in CMS such as Drupal which uses the term 'entity' and 'entity type' for its content, and since Clickup is a hybrid this would be a great improvement.