What relationships are not allowed?
A company cannot be its own parent.
A company cannot be placed under one of its own sub companies, at any level. This prevents circular hierarchies.
When you join two existing hierarchies, for example by placing one company's whole hierarchy under another, the combined structure is checked for circular relationships.
Can I delete a company that has sub companies?
No. A company cannot be deleted while it still has sub companies. Unassign them, or move them under another parent, first.