Hierarchies in BW

I am a technical consultant (ABAP) and want to know the use of hierarchies in BW.

We have a requirement in which there are a certain G/L accounts which have to be categorized according to their type, i.e. , whether they are fixed or variable account. The account type might change monthly which has to be maintained. Based on this, the reports will be developed. The customer does not want to create a custom table in R/3 for the same and is recommending the use of hierarchies in BW.

Would it be advisable to maintain the flag of fix and variable ratio as a characteristic of the account dimension or would it be better to create hierarchies and if so, how?

Please advise.