I doubt very much you can write to SRCEAPPL. You should setup an Optional Field (if you have the module, very much worth the investment) at the invoice header level and predefine the allowed values in the optional field configuration.  The values will flow through all of the areas where an AR invoice can be found in the database and can also flow all the way into the GL if necessary.

Invoice Header Option field values are not available on ARPostedDocuments WebAPI Payload and we need a field that is available.We can actually update SRCEAPPL using normal Batch Import, our concern is any risk in updating it.

Oh, so you want to send a value and be able to retrieve from PostedDocuments later on?  I didn't get that from your question, I thought you simply wanted to send a value.  I have no experience in using SRCEAPPL, I thought that it was a reserved field for established 2 letter codes for Sage core or fully integrated modules only.  You might have to ask Sage the question.