Here is some code that will process your string container into lines of records that you want but you might want to change a few things to get this working with your records after converting your string container to a simple csv file you can then map it to your record mapper and process it any how.

Class TestEnvironment.Custom.GENERAL.ContainerString Extends (%Persistent, %Populate, %XML.Adaptor, %ZEN.DataModel.Adaptor)
{

Property Mystring As %String(MAXLEN = 32000, XMLPROJECTION = "CONTENT");

ClassMethod Import()
{
    // Create an instance of %XML.Reader
    Set reader = ##class(%XML.Reader).%New()

    // Begin processing of the file
    Set status = reader.OpenFile("C:\TEST\test.xml")
    If $$$ISERR(status) {do $System.Status.DisplayError(status)}

    // Associate a class name with the XML element name
    Do reader.Correlate("StringValue","TestEnvironment.Custom.GENERAL.ContainerString")
    
    // Read objects from xml file
    While (reader.Next(.object,.status)) {
	    
	   
        Write object.Mystring,!
        
//check the length of the string
         set lstr=$length(object.Mystring,",")
	    write "The length of incoming string is : "_lstr
	    if (lstr>28)
	    {
//divide by 28 for your processing
		    set div=lstr/28
		    if (div=2)
		    {
//assign your strings to something and write to file to map your records
			    set newstrObject=$piece(object.Mystring,",",1,28)
			    set secNewStrOJect=$piece(object.Mystring,",",29,56)
			 }
			 elseif (div=3)
		    {
			    set newstrObject=$piece(object.Mystring,",",1,28)
			    set secNewStrOJect=$piece(object.Mystring,",",29,56)
			     set thirdNewStrOJect=$piece(object.Mystring,",",57,84)
			     
			     write "this is the string cut1 :"_newstrObject,!
			      write "this is the string cut2 :"_secNewStrOJect,!
			       write "this is the string cut3 :"_thirdNewStrOJect,!
			 
		 }}
    }
    
    // If error found during processing, show it
    If $$$ISERR(status) {do $System.Status.DisplayError(status)}
}


}

hope this helps