The ID of a micropost that is considered the 'parent' of this post
$target
—
mixed
The "target" of this post; may be a data object (ie context of the post) or a user/group
$to
—
array
The people/groups this post is being sent to. This is an array of - logged_in: boolean (logged in users; uses a system config setting to determine which group represents 'logged in' - members: an array, or comma separated string, of member IDs - groups: an array, or comma separated string, of group IDs