Physical contacts enable you to calculate real contact interaction between two different bodies of the domain as well as self-contact of different faces of one body. Unlike for constrained Contacts those faces are not just connected via linear relations but the actual contact forces are calculated.
In order to enable a Physical Contact interaction you have to define contact pairs of faces or face sets. For those faces the distance between each other is tested during the simulation and if they get in contact the interaction forces that prevent those faces from interpenetrating each other are taken into account. As those forces only occur in case of contact the interaction is a nonlinear phenomenon and thus only applies for nonlinear analyses.
The solution of the nonlinear contact has to be done via a specific contact solution method.
The contact solution method has to be chosen in a global sense in a simulation setup. Of course you can define different properties for different contact pairs, but it is not possible to switch between for example penalty and lagrangian contact.
Please be aware that one face can not be a slave face of different contact definitions simultaneousy. This also applies for slave faces of Contacts.
There are some general rules that help you to decide which of the contact faces or sets to choose as master and which to choose as slave entities. Of course those rules do not work in every case but they provide a good starting point. Choose as slave entities, face(s) that are
considerably smaller than their counter part.
strongly curved compared to the other part of the contact pair.
not as stiff as the other part, especially if the other part is even rigid.
that have a considerably finer mesh than their counter part.