Board-relation column settings_str description

I got info about the board’s columns through the API.
There was a column with the type board-relation.

{
  "id": "connect_boards",
  "title": "Deals",
  "type": "board-relation",
  "settings_str": "{\\"boardIds\\":[2816754151,2816754217],\\"boardId\\":2816754151}"
}

There are “boardIds” and “boardId” in the settings_str.
What do they mean and what is the difference between them?