The Name 'Special GL' tells it all. Firstly the Special GL is nothing but a Reconciliation Account which holds the entries which are of special Nature..
Example: If I am dealing with Vendors. Generally transactions which are shown in Case of Vendors Will be any of :
3. Retention Amount
4. Security Deposits
We have to show each of these above components separately in the Balance Sheet, which is for the obvious reason that there is lot of difference in the nature of each component and each component shows different effect on Balance sheet in their own way.
Now we can assign a Reconciliation Account in Vendor Master Data, which in almost all the cases will be a Liability GL Account, as the primary nature of the Vendor is a Liability.
Taking this Reconciliation as the Link, we assign the Other GL accounts with the help of the 'Special GL Indicator', where each indicator represent each special natured component. Example: Lets us say Special GL Indicator 'A' indicates advances. We link a GL account 'YYYYY' - Advance GL account with the 'XXXXX' - Main Reconciliation Account against the special GL Indicator 'A'. This will help the user to post the amount into desired GL account with the help of Special GL Indicator.