What is a step page what role does it play in an activity?

Every step in an Activity runs against a page. The Step page setting on the step tells Pega which page that is. Get this wrong and the step reads or writes the wrong data, or fails with a "page not found" error.

The rule of thumb

If the Step page field is blank, the step runs on the activity's primary page. That is the page the activity was started on, for example pyWorkPage when called from a flow.
If you type a page name, the step runs on that page instead, and every property reference such as .Amount means thatPage.Amount.

Where step pages come from

Source	Example
Created by an earlier step	Page-New makes a page called `NewCustomer`
Loaded from the database	Obj-Open with Step page = `CustomerPage`
Passed in as a parameter	A step page set to a page name held in `Param.PageName`
An item in a list during a loop	Each embedded page when iterating `.Items`

A worked example

A bank activity must open a customer record and copy the email address onto the current case.

Step 1: Obj-Open-By-Handle with Step page CustomerPage and the customer key as a parameter. It loads the customer into a new page called CustomerPage.
Step 2: Property-Set with Step page blank. The primary page is the case, so .Email = CustomerPage.Email writes to the case.
Step 3: Page-Remove on CustomerPage to clear it from the clipboard.

If you had set the step page of step 2 to CustomerPage by mistake, the email would be copied onto the customer itself, not the case.

Common mistakes

Leaving the step page blank when the property lives on another page.
Forgetting that Param. and Local. variables are not pages. They do not depend on the step page.
Leaving temporary pages on the clipboard. Remove them with Page-Remove when you finish.

Interview tip

A good answer: "The step page is the page a step operates on. When it is blank, Pega uses the primary page. I set it explicitly when I work with a page I opened, created or looped over." Related: Parameters vs local variables and the Activity label.