In an activity ProcessOrder, the fourth step invokes another activity CheckInventory. The step page for step 4 is specified to use a page named OrderInfo that is listed the Page & Classes tab. When the CheckInventory is executed by step 4 of the ProcessOrder, what is the primary page for ProcessOrder execution?
When you give a page as a step page, the step will be executed in the context of the step page. This means that, if you are referring a property without page name (something like .propName), it will use the step page as the context (stepPage.propName). And if you are calling another activity, the step page's class will be used to look for the activity. But if you are using the keyword "Primary" to refer the property name, it will consider the primary page on which the current activity is called.
In your case, Context for step4 will be OrderInfo page. But if you use any property with Primary keyword, then it will be primary page on which ProcessOrder is called.