Every thermodynamic system is described by a thermodynamic potential obeying the above relations.
If one knows the thermodynamic potential for one contact, the thermodynamic potential for all other contacts can easily be calculated.
All thermodynamic restrictions for a thermodynamic system are automatically fulfilled.
The thermodynamic restrictions depend on the kind of contact, i.e. on the interactions between the system and the surrounding.
For all this interactions the thermodynamic potential can easily be calculated when just one thermodynamic potential for one special contact is known.