Purchase Business Setup (PBS) gives you one online function to set up rules and controls for the whole purchase activity in your organization. You may need, for example, to set valid accounting periods or rules for budget checking.
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The following information is defined in the Purchase Business Setup:

The ledger files to write transactions to. The calendar dates when transactions can be processed. Certain rules and controls in the modules. Default information to aid operator input.

The purchase module is defined using Purchase Business Setup (PBS) which contains the following:

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System Tab
Business Unit
The Business Unit for ledger transactions to be posted to. This allows posting to another Business Unit. If you leave this blank postings are made to the current Business Unit.
Revision History Required
This box is checked if a revision history is required. Revision records are created when significant changes are made to an order or invoice, line or header.
Currency Analysis Directory
The analyzable entity used as the default currency code on a Purchase Order line, if none is supplied by the Purchase Type or entered by the user. This is always an Analysis Code. In the Analysis Directory specify where the code is held, for example, on the Order by Address. This field cannot be left blank if the Transaction Currency field has been activated in the Business Unit Setup.
Transaction Analysis Dimension
This identifies which of the ten possible analysis dimensions attached to the above analyzable entity holds the currency code. This means that by holding a currency code as an analysis dimension against an address, for example, the correct currency is automatically selected whenever that address is used. If you leave these entries blank, or the code is blank, the Base Currency is used instead.

Batch or Online Posting
If you are serialized to use SunSystems Financials, this field indicates whether posting is carried out online (after posting requested) or by batch (when requested by Financials).
Error Suspense Account Value 1-3
Indicates the Error Suspense Account to be used to correct imbalances for each value. Used when posting is selected. See Dealing with Errors in Ledger Import for balancing rules.
Current Period
The current accounting period to be used for the Purchasing module. This can also be amended using the Change Period (CP) function. See Changing the Current Period for more information about the implications of changing periods.
Budget Ledger Code
If you are serialized for SunSystems Financials, this field specifies the Budget Ledger code (B to K). This is used if Budget Checking is in use to determine what budget is available for an order or invoice. It cannot be different to the code defined in the Business Unit Setup.
Commitment Ledger Code
If you are serialized for SunSystems Financials, this field specifies the Budget Ledger code (B to K). All commitment postings are made to this ledger. It cannot be different to the code defined in the Business Unit Setup (BUS).