The Associated Registers tab for any contact or entity, displays where the contact is connected to another entity via a register (a reverse look back at the registers). You would see for example an individual person where perhaps they are a Director of Company or Beneficiary of Trust. All the register types are available in the Associated Registers tab as potentially a contact could be in a register for any entity type.
The registers themselves are not displayed on every entity type, only those which are relevant to the administration of it. For example, you would not see the Register of Beneficiaries on a Company entity type, or Director on a trust entity type as these concepts do not exist. This is why the associated registers differs from the registers tab and shows all the registers available across all entity types.