The Client_Matter_ID is the 24th field of each record in the Ledes98b format, and this information is typically entered separately from your normal Matter record.  To enter this information in the system, open the Orion EDI Billing module and click on the Insurance Case Data Button.  Enter the appropriate Client-Matter Number and you will see at least one field of information you are required to enter (if more than one field, typically the first field).  Enter the Client_Matter_ID as assigned either by your Client or the third party company and click SAVE.  Information entered in this field will be output to the 24th field in each record.  Re-create your EDI bill and re-submit it.