"Contact Details" In case of a tax agent, it is used for tax forms, as the ATO requires an agent's name. , Also, it is  for e-signing forms/SPFR by agents.

General Contact Details. In this case we are filling the details for a company and so the signatory must be noted.

"Settings > Contact Details" for Individual entity -

"Settings > Contact Details" for Company, Partnership, SMSF, Trust, and Partnership.

Note that Primary Contact/Signatory is the person who receives, signs the tax return.

User (tax agent) need to "Untick" the box if he wants the tax form to be sent to himself as agent of the client.

For FOREIGN addresses State and Postcode are not required on tax forms (also not required to fill under Settings>Contact details). ATO requirement is that postcode is  maximum 4 characters long. Workaround for this kind of issue is to left empty the said fields (State and Postcode) and add this either Line 1 or 2.

Related article: 

Address Option - Tax Agent or Client Additional Email Address