To purchase content or restore using in-app purchase, the end user must sign in to Nokia Store. During the time that the sign-in is valid, the end user can purchase new content or restore previously purchased content without having to sign in again.
In addition to prompting the end user for their password, Nokia Store may also prompt for their user name. This may occur, for example, if the end user has not allowed the device to store this data, or is signing in to Nokia Store for the first time on their device.
In a purchase scenario, the sign-in to Nokia Store is handled transparently to the application. During the payment part of the scenario, the in-app purchase API simply prompts for a sign-in or proceeds to the purchase confirmation, depending on whether or not the user is currently signed in to Nokia Store on their device.
In a restoration scenario, whether or not the end user is signed in might make a difference in your design choices. You could design your application so that if the end user is signed in, the application tries to silently check if any restorable products are available. This way the application can be helpful to the user and display restoration options to the user right away, or it can even silently restore them without requiring any action from the user. And you could design the application so that if the end user is not signed in, they will not automatically be bothered with a password prompt. Information about restorable products would just not be available to the user, unless they specifically select a UI option to find out, and at this point a sign-in would be prompted.
For concrete examples of UI designs for restoration, see an example design where the user is signed in, and an example design where the user is not signed in.