In the master model object window you can access the master parameters by Parameters|Masters which opens the following dialog:

You can define up to 6 master parameters each of which has a name, a value and the number of digits that is used to display the master parameter's value. In the formulas of the slave parameters you can refer to the master parameters by C1, C2, ... and C6, respectively. The master parameters appear as fit parameters of the master model objects.