It looks like the Source field is associated with the APPrintCheckDetail DAC and can have one of three values:  A, P, or T.  I only reviewed one customer’s data but it looks like the source field will have a value of T if the check is a prepayment and it is not applied to an invoice.

You can review your data to determine what a particular check you are printing has for a source value by going to generic inquiry screen (SM208000) and adding the PX.Objects.AP.APPrintCheckDetail table.  Then click on the table link to open the schema browser.  In the upper right corner of the schema browser there are a couple of links.  

Source Data will show you the data records and you can filter on the source field to find checks that have a source value of T or find the value of the source check you are printing.

DAC Query is interesting in how it populates the various fields used in the report based on the source field value.  Here are a couple examples in the event you can’t get to these screens.

Here the AdjdDocDate is set based on the source field value:

( CASE
WHEN (EAPPrintCheckDetailWithAdjdDoc_APPrintCheckDetail].kSource] = 'T')
THEN 'APPrintCheckDetailWithAdjdDoc_APPrintCheckDetail].kOutstandingBalanceDate]
ELSE ISNULL(>APPrintCheckDetailWithAdjdDoc_APInvoice_APRegister].eDocDate], ]APPrintCheckDetailWithAdjdDoc_POOrder].POrderDate])
END ) AS rAdjdDocDate],

Here the OrderBy is set based on the Source field value:

( CASE
WHEN (EAPPrintCheckDetailWithAdjdDoc_APPrintCheckDetail].kSource] = 'A') THEN CONVERT(SMALLINT, 0)
WHEN ()APPrintCheckDetailWithAdjdDoc_APPrintCheckDetail].kSource] = 'P') THEN CONVERT(SMALLINT, 1) ELSE CONVERT(SMALLINT, 2)
END ) AS rOrderBy]

I hope this helps.

@johnw51 your answer seems to have set me down the right path for figuring this out, as its a bit of a case-by-case situation the rest is really up to me. thank you for the answer it has been very helpful.