The activity would typically be the one from which you are calling the ApplyModel function.

I can see in the java code for that function that it passes the step page of the activity to the applyModel method, and it also uses the activity as the java object from which to all the applyModel method.

Here's the java for that function:

ClipboardPage myStepPage = ThisActivity.getStepPage();
ThisActivity.applyModel(myStepPage, null,ModelName);
return myStepPage;