I think that you are working on the AX 2009 , there is functionality called Dimension set, Dimension set hierarchy and Dimension set rule in 2009.
As you know that during posting any documents /journals like Invoice journal / Payment journal User will attach the dimensions , then post the transactions. If user do not attach the dimensions system will throw the error.
The functionality can be achieved using the Dimension set and hierarchy in AX 2009 , .
In Microsoft Dynamics AX 2012, account structures replace what was accomplished through dimension sets, dimension set hierarchies, and dimension set rules in Microsoft Dynamics AX 2009.
Account structures consist of the main accounts and can include any financial dimensions as segments. An account structure is used to define the valid dimensions
which, together with the main accounts, form a chart of accounts. You can create multiple account structures that can be mapped to the same legal entity or company.
You have the dimension like Business unit , depatment , Cost centre. Your client told if user posting any transactions which will effect the BS accounts then the Business unit is manadatory , which will effect the P&L accounts the Department and cost centre is mandatory.
For example, assume that all of your P/L accounts in Microsoft Dynamics AX 2012 have the main account range between 40000 and 99999 and all of your B.S. accounts in Microsoft Dynamics AX 2012 have the main account range between 10000 and 39999. You could create two account structures: one called ‘Account Structure - P&L’; and the other called ‘Account Structure - B.S.’. You could then map all of your 40000 - 99999 main accounts to the ‘Account Structure - P&L’ account structure and all of your 10000 - 39999 main accounts to the ‘Account Structure - B.S’ account structure. You could then select which financial dimensions could be assigned as segments to the ‘Account Structure - P&L’ and ‘Account Structure - B.S.’ account structures. For example, you may want to have validation rule setup for the Department and Cost Center financial dimensions for the P/L accounts assigned to the ‘Account Structure - P&L’ account.
Advance rule :
Advanced rules can provide for additional dimensions outside of the account structure.
For example for the P&L accounts , when the Department is either the Dep1 or Dep115 Department financial dimension value then you could select the C1 as Cost center financial dimension value. If you were to select a different Cost center financial dimension value, the validation will fail.