Classes of transactions & account balance

They don't require formal definition - they are generally understood terms - an account balance is just that - any balance on a (general ledger) a/c that will be a balance in SoFP. Class of transactions - transactions of the same type/class - e.g. sales is a class, purchases is a class, etc.

Ok, sir how about like water & electricity expense? Does it consider as one class of transaction? OR the total administration expense only consider as one class of transactions? Thanks

If you consider the financial statement assertions it wouldn't make sense to consider these independently for "water and electric expense" and separately again for "rent and rates", etc, etc. So a class is what you can put together as "like" items - and in that respect I would say "like" should include how the transactions are processed. So I would distinguish purchases (of raw materials/finished goods) from expenses (collectively). Even though all may be recorded in a purchase day book/purchase ledger, purchases would also have inventory-related documents and records. Bear in mind that additions to non-current assets would be a separate class of transactions (e.g. they require different authorisation and they are also recorded in an asset register, etc) and disposals of non-current assets would be a separate class (distinguishable from sales of goods).