Mylist entries can be placed into the following states. You may if you wish turn on overwrite protection and set whatever state you wish, but if you do not the website will automatically set one of these states as per the conditions below.
The default state. This will always get overwritten by the automatic filling. it's mainly there for manual adding and fallback.
You have episodes of this anime in your mylist, but have not watched any of them.
You have episodes of this anime in your mylist, and have marked one of them watched within the last 3 months.
You have episodes of this anime marked watched in your mylist, but did not mark any of them watched within the last 3 months.
You have all episodes of this anime in your mylist, and have marked all of them as watched.
You do not intend to continue watching this anime. This state will not be set automatically, and if you set it manually it will not be overwritten unless you mark the entire of the anime watched.