Control accounts are the accounts a subledger owns: Trade Debtors belongs to your customers' invoices and receipts, Trade Creditors to your suppliers' bills and payments, Stock to the warehouse, the GRNI account to goods received but not yet invoiced. Their balances are never typed or journaled into existence — they are simply the sum of the detail underneath, and the application marks them with the Control account flag so nothing else can touch them (the next section explains the refusal you will meet if you try).
Contra accounts run the other way to their group: accumulated depreciation sits inside Fixed Assets but subtracts, and statements show it as "Less: Accumulated depreciation" rather than pretending it is an asset of a negative kind. The Contra account flag is what earns an account that presentation.
A handful of accounts are flagged for special jobs: exactly one retained-earnings account (where year-end results land, 7.6), the FX gain/loss account (where exchange differences land, 7.11), and — if you manufacture — work-in-progress and manufacturing-variance accounts. The flags, not the names, are what the posting engine looks for, so renaming an account is safe, and moving a flag is deliberate.