Account families describe the relationship between different business entities or specific sites within a corporate family. When one business entity has financial or legal responsibility for another business, it creates a headquarters to the subsidiary relationship. These relationships are shown in the data with three fields:
Account ID: Gives an account at a specific location a unique identifier.
Parent Account ID: Indicates the parent entity for the account.
Ultimate Parent ID: Indicates the account at the very top of the corporate hierarchy, which owns all of the subsidiaries.
Options to treat account families
When you create territories, select how account families are treated from the available options:
Note
When you create segment hierarchies from the parent, only the records that belong to a family that are associated with that parent are included. If you have accounts that belong to the same family in other nodes within the hierarchy, they are not included..
Don't reassign children to the same segment
Excludes any family relationships and matches all the accounts based on the specified segment hierarchy rule (unique values or buckets). All the accounts, regardless of account family will be matched and any record that matches the rules will be assigned to the respective segment. 
Assign all children not selected in the current scope to the same segment as the parent
The parent account is evaluated against the rules and if there are any child accounts directly related to the children (as identified by the Parent Account ID field), they will be moved to the segment where the parent will be assigned to. The scope includes not only the direct children but also the entire children hierarchy under that parent.
Note
Only the children of the parents that are matched will be moved with the parent. If a parent account is not available in the selected segment, each sibling will be individually evaluated against the segment hierarchy criteria and assigned independently.
Assign children not selected in the current scope that match the following criteria to the same segment as the parent
The parent account is evaluated against the rules and specific criteria determines if there are any child accounts that qualify to be moved with the parent.
This is common in situations where there is a need to keep all accounts that belong to an account family within a particular region (all US accounts of a family), but want to independently evaluate the child accounts based on specific criteria.
Note
Select the correct field and type options in the account families menu to keep the number of territories created within a manageable number. When you create segments with specific criteria assigned for children accounts, it's easy to fill out the primary field as segmentation criteria, but it is important to make sure not to create unique values for high numbers of records as it will only crowd the segment hierarchy.