Each production plan task involves one item, which can be either a recipe or a custom task:

If the item is a recipe, the item_class will be recipe and the item_id will be the ID of a recipe. This applies to tasks specific to a recipe. In this case, the quantity_to_produce field is required.
If the item is a custom task, the item_class will be custom, and the item_id will be the ID of a custom task. This is used for more general tasks and is not tied to any specific recipe. In this case, the quantity_to_produce field is not required.

In short, the item_class field helps identify what the task relates to.