What causes ASTA in F03b13 to be a value of 1 after the auto receipt program runs?

Recently, we've had two unapplied receipts that should up in the Standard Receipts Entry application, but when you try to select it to drill into the detail, it says 'Record In Use by Auto Receipts'. I identified that the ASTA field in the F03b13 was set to a 1 rather than a 2 which I typically see for unapplied receipts. Once that was changed, we could look at the receipt.

1. What causes the ASTA to be set to a 1 causing this error to arise? I found that the R03B551 sets the value but didn't understand the difference between the meanings 1, 2, 3, etc.