The ID number for the entry as per MyAnimeList. This can be found at the end of the URL.
For example, the first City Hunter anime has the URL http://myanimelist.net/anime/1470, with the ID being 1470. Please make sure you are linking the correct anime or manga ID as this is used to accurately import lists.
The misc section for the 'Ao Buta' anime
Please don’t add a MyAnimeList ID if the entry is unapproved. You can normally spot unapproved entries as they will be on zero user lists you'll receive a warning message if you try to add one to your own list.
Each submission to MyAnimeList generates its own ID, so it’s possible that the ID for the entry submitted to AniList will not be the ID of the entry that MyAnimeList later approves, as some series will have multiple unapproved versions on the website.
Incorrect IDs mean users using the import feature will receive an error or incorrect entry. Unfortunately, MyAnimeList can often take a long time to approve works (more so for manga), as such you may not be able to link entries for a while.