Line item display is set for those accounts which are specifically required to display both side of a transaction in its report display i.e. debit and credit side of a transaction. When you post a transaction to gl account which has been set to line item display in its ledger master, system will store it in a special index table in addition to storing it in the documents. When a user tries to access these line items, system will look for both the balances and individual line items. Transactions for these accounts are stored in two different places by which sap needs additional storage and system time.
Since line item display takes up additional system resources, sap suggest us to use it only for those accounts which require this functionality specifically. If there is no other way to get into the document display of a transaction then this functionality should be used while creating general ledger master.
Assets, Vendors and customers are managed through different sub ledgers having a link to the main general ledger. While creating these master data we first create the reconciliation account and then assign that to each master data.
Based on your requirements different reconciliation accounts are created. While posting documents to these master data, line items are updated at the master data level in their respective sub ledgers from which balance entries are posted to main ledger through reconciliation account.
This is an inbuilt feature in sap which does not require you to do anything from your end. To make the system efficient and to save storage space in sap, it has been developed in that way. For these types of account, you can access line item display from the individual sub ledgers and balance display from reconciliation accounts.
As transaction or detail document display can be seen from sub ledgers, to save system storage space and time, sap did not allow reconciliation accounts to have line item display. That is the reason while creating a reconciliation account in general ledger master by using transaction code FS00, system will through an error message that, reconciliation account does not require line item display.