Access attribute is only enabled in Knowledge Based System, used for control both of read and edit rights of the post, usually it contains one or more user groups.

Depended on user roles, user groups are pre-defined by administrator, used to separate editors to different groups. Then editors can assign their groups to posts. So only editors in the same group can read/edit their posts. But if one post does not assign to any user groups, it means this post can read/edit by any editors.

Read and edit rights of the user groups are pre-defined by administrator, you can move mouse on the info link to check the details. Below example means all login user can read this post but only login users in the same group can edit this post.