Normally LinuxMCE allows everyone in the house to access everything. There are 2 ways to restrict this. One is by requiring certain users to enter their PIN code before they can use certain orbiters. The other is by only allowing certain users to control the devices in certain rooms.
Let's say that you are the parent and you do not want your children to be able to control the television in the master bedroom. You would uncheck the boxes for your children under the master bedroom to indicate they cannot control that room. Then you would check the box for you under any Orbiters that your children had access to. We'll assume the children have access to the Orbiter in your living room, but not the one in your bedroom.
In this case, you can control everything in the house from the Orbiter you keep in your bedroom without entering any pin codes. But, when you go to use the Orbiter in the living room you will need to enter a PIN code so LinuxMCE knows it's you. Your children could use the Orbiter in the living without entering a PIN code, but would not be able to control any of the devices, or more accurately execute any of the scenarios, in the master bedroom.