There are two possible states:
- This layer is a normal layer (with discontinuous cels): new cels will be created unlinked (cels are copied).
- This layer is continuous: new cels are created in a linked way.
Generally you will prefer continuous cels for background layers (with static content), and discontinuous mode in layers that have a different cel on each frame:
This option can be switched depending on the specific situation you are working on. Modifying the option doesn't change current cels, so it only affects future operations.