You will only see the 'Send to GL Description 2' option if you are using Adagio Ledger 90A. It supports 2 descriptions for GL batch entries, thus you can send more information.

If you use %des as the User Defined code, the information flowing to GL depends on the account. For the revenue and cost accounts, it will be the Item description. For the tax account, it will have 'Tax for doc XXX'. Control/clearing accounts will be referenced as such.

%des is the Default value for the 2nd Send to GL Description field.