Hi Tim,
As much as possible, use the same fields as you would use for a recipient, depending on the personalization you need to include in your push notifications. We have a few documents coming shortly about push notifications, I do not know the exact content but they should greatly help, stay tuned!
I think the minimal information would be targeting information used to query the recipient (age? name/firstname? country?), as well as the mobile app ID, as you'd be sending the notification through the app...
Let me know,
Florent