For a contact on SimScale, you always have a master and a slave assignment. During the calculation, the nodes of the slave surfaces are restricted in their movement by the deformation of the master surface.
There have been a number of questions and comments on FEA Public Projects regarding assigning master and slave faces when setting up contacts. Here’s a list I’ve compiled so you can get all the hints in one place 
1..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. 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.
2…Simulation time for the physical contacts mainly depends on the size of the slave surfaces and almost negligibly on the size if the master surfaces (this means number of nodes). Assigning large surfaces to slave means that the simulation will require more memory or time to solve the contact. Therefore, it’s always a good idea to assign small surfaces to slave or otherwise partition the surfaces.
3…Each face can only be a contact slave once (more accurately: each node can only be in a contact slave once). On the other hand, a surface can be a master in multiple contacts.
4…In a contact analysis with friction one should assume that the simulation time will increase by at least 2 times. If you have a lot of iterations, changing the contact algorithm might also bring speed improvements, usually Lagrangian contact with the full newton method is the fastest, but you should note that stability is not as good as Penalty with the fixed point algorithm.