What is a subledger
The subledger, or subsidiary ledger, is a subset of the general ledger used in accounting. The subledger shows detail for part of the accounting records such as property and equipment, prepaid expenses, etc. The detail would include such items as date the item was purchased or expense incurred, a description of the item, the original balance, and the net book value. The total of the subledger would match the line item amount on the general ledger.

There are instances when items will go directly to the general ledger without any subledger.These items will be linked to your balance sheet but not to your profit and loss statement.

What is SLA in R12

Oracle Subledger Accounting is a rules-based engine for generating accounting entries based on source transactions from ALL Oracle Applications.
Subledger Accounting is a Service, not an Application or a new module.So, SLA forms and programs are embedded within standard Oracle Application responsibilities (e.g. Payables Manager).There are no SLA responsibilities.

SLA provides the following services to Oracle Applications:

• Generation and storage of detailed accounting entries
• Storage of Subledger balances
• Subledger accounting entries (with Bidirectional drilldown to /from transactions)
• Subledger reporting

Features of SLA

• Replaces various disparate 11i setups, providing single source of truth for financial and management analysis. It introduces a common data model and UI across subledgers
• Allows multiple accounting representations for a single business event Optionally Post subledger accounting entries to Secondary Ledgers Resolves conflicts between Corporate and Local Accounting Requirements
• Highly granular level of detail in the Subledger accounting model retained
• Accounting Model separate from Transactional Model
• Catering to custom requirements of accounting of transactions in Subledgers
• Accounting created in Draft or Final mode : 

1. Draft: Review Report, Correct errors
2. Final:Transfer to GL, Post in GL

Benifits of SLA
1. The flexibility of the accounting rule setup allows meeting different requirements in different legislative, geographic or industry contexts.
Assuming operations in multiple countries, each with its own legal requirements and accounting standards, you are able to define a setup to meet each of the requirements. SLA allows for multiple accounting requirements for a single transaction or business event.

2. Increase transparency and enable full auditability of the transaction and accounting data through the new data model
In 11i, accounting information and the tie back to the underlying transaction is maintained differently for each module; and sometimes not at all!
Different subledgers/modules have their own model as to how and what they capture in terms of accounting data.  Some allow capturing more details and some link between the journals and the underlying transactions.  Some do not have the same flexibility or detail.  This causes difficulty and inconsistency in reporting for auditing, reconciliation or whichever purpose across modules.

Another second benefit that SLA brings in R12 is the ability to retain the full link between trx and accounting data for all modules, and thus allow auditability.  The accounting SLA creates is strongly tied to the underlying transactions.

3. Another benefit is the support for business flows in accounting within and across modules.  Basically, in flows involving multiple transactions whose accounting need to be tied together, SLA provides standard features to recognize multi-transactional flows.  This is very important in especially reconciling accounts inter-transactional accounts.