ROH -3000 is used for rawmaterials in standard SAP. It actually depends upon the combination of Valuation class, Account category reference and Mterial type...which help system to determine the GL account during the document posting.
Check transaction OMSK and OBYC to understand the behaviour of the system.
We use the valuation class for the purpose of valuation of valuated materials e. g. Raw Materials- ROH or Trading Goods - HAWA. This helps in determining the G/L accounts in the acc. determination process, so that the values can be posted to the right G/L account during invoice receipt.
The valuation class is a group of materials with the same account determination.
The valuation classes allowed depend on the material type. Several valuation classes are generally allowed for one material type. A valuation class can also be allowed for several material types.
The link between the valuation classes and the material types is set up via the account category reference.
In the standard system, an account category reference is created for each material type. The account category reference is, in turn, assigned to precisely one valuation class. This means that each material type has its own valuation class.
3000, in general, is used as valuation class for raw materails.