Set up groups with parent-child hierarchies in the approval group.

[Community Alums]

Set the parent-child hierarchy when creating a group.
When using the user criteria, setting a parent group gives the child group the same privileges as the parent group, and members of the child group can also view the knowledge article.

On the other hand, if you set a parent group as the owning group for a knowledge article, an approval request will be sent only to the members of the parent group and not to the child groups.
Is there any official documentation that mentions this specification?

[Mark Manders]

These are two unrelated functionalities that are just both with groups and both with knowledge. User criteria are for who can see what and contribute to. That's a complete different thing than owning what can be seen.

Having an 'ITIL' role-group and putting child groups under that for different assignment groups, grants the users the right to see incidents, but I still can't assign an incident on group a to a member of group b.

The notification is set to send it to the owner group of the article, not to the owner group and it's childs. This is not a specification, it is just how the notifications are setup. 

[Community Alums]

If I set up a parent group as the owning group, why isn't an approval request sent to the child group?
If a group has a parent-child relationship, the child group should inherit the parent group's permissions.
I am also looking for official documentation on this.

[Mark Manders · Accepted solution]

As I already said: these two are unrelated. Permissions on access to articles/bases have nothing to do with approving articles. The child group inherits the permissions, so the users will have the same access. That doesn't make them approvers. It's apples and oranges. Yes, both are fruit (groups) and grow on a tree (have to do with knowledge functionality) but are completely different topics.

Child groups never grant permission to approve something for the parent group, unless you create it custom. There isn't documentation on something that doesn't exist. This is the doc about knowledge ownership groups: https://docs.servicenow.com/bundle/washingtondc-servicenow-platform/page/product/knowledge-managemen...