I have the following doubts about product_packaging model:

1. What is sequence and when is that field actually used?
2. Why some entries lack product_tmpl_id (which if I am right they correspond to the packages generated via menu Inventory -> Configuration -> Package types) and others have a reference to a given product_tmpl_id when created directly from product form?
3. (relates to 2) Which is the difference between them?
4. (relates to 2) When are they used and which is the functional need to have some referencing a given product and others not referencing any specific product?

select * from product_packaging;
 id | create_uid |          name           | sequence | qty  | product_tmpl_id |         write_date         |        create_date         | write_uid
----+------------+-------------------------+----------+------+-----------------+----------------------------+----------------------------+-----------
  1 |          1 | Caja de 1000            |        1 | 1000 |                 | 2017-09-04 03:39:13.684079 | 2017-09-04 03:39:13.684079 |         1
  2 |          1 | Pallet de Cajas de 1000 |        1 |   20 |                 | 2017-09-04 03:40:05.973295 | 2017-09-04 03:39:51.296957 |         1
  3 |          1 | Caja de 1000            |        1 | 1000 |             246 | 2017-09-05 02:36:15.702681 | 2017-09-05 02:36:15.702681 |         1