The Paid field in the event app references the participant's Registration Fees, not their Participant Status. The field is intended to help determine when a participant hasn't yet been linked with a gift in The Raiser's Edge. The app calculates the App's Payment field as follows:

Gift Amount Total - (Total amount of linked gifts in uncommitted batches + Amount Paid field) = Unpaid amount in the Event App

Unpaid Participant Record

The participant will show up in the event app as Not Paid if:
  • The total of their Gift Amount column on the Registration Fees tab is greater than the sum of the Amount Paid field and uncommitted linked gifts in batches
The participant will show up in the event app as Paid if:
  • Their Registration Fees tab has no Registration Fees listed
  • Registration Fees are listed with 0 Units
  • Registration Fees are listed with a $0 Gift Amount
  • The sum of the Amount Paid field and uncommitted linked gifts in batches is greater than or equal to the total Gift Amounts listed in the Registration Fees grid
NOTE: Linked gifts in uncommitted batches and pledge gifts (whether they have a balance or not) are both considered by the event app. The Amount Paid field on the Registration Fees tab of a participant record does not increase based on unpaid pledges -- only paid ones.

For example: 
A participant has registered for 3 units with a total Gift Amount of $300, and has a $100 gift linked to that event on their record (as in the screen shot above). They will be in the app as Not paid ($200) to reflect that they still owe $200 of the $300 expected. So while the participant has a value in Amount Paidit is not yet the full amount expected toward registration fees, so $200 is Not Paid.

Unpaid Participant in App

If I check in that participant using the Event app and select to Pay Later, the app creates an unpaid Pledge gift for the remaining amount in the selected batch for the event. Selecting to Check in and Pay in the app will always make the participant show as Paid, because a gift for the remaining amount is automatically created in the batch. 

Pledge Applied to Event in Batch

The Paid field in the RE Event App DOES include uncommitted batches linked to the event, so lists the participant as Paid
Amount Paid on the participant record in Raiser's Edge does NOT include uncommitted batches, so the Amount Paid still lists as $100, and the pledge is not included when clicking the View Gifts button. 

After the batch is committed, the pledge is listed when clicking View Gifts on the Participant's Registration Fees tab, but the pledged amount is not included in the Amount Paid field. This is because The Raiser's Edge distinguishes between unpaid and paid pledges, while the Event app is intended to identify participants with fees unlinked to gifts.