Twice in the adjustments we credited a strange account instead of the asset itself. A contra account is paired against another account and carries the opposite normal balance. That way the books can show a net figure and still keep the gross information. You've met two of them.
Why is accumulated depreciation a contra account?
Accumulated Depreciation is a contra-asset to Equipment, and it's credit-normal. We could have credited Equipment ₹4,000 directly, and the net effect would have been identical. But then the balance sheet would just say "Equipment: 2,36,000," and you couldn't tell a fleet of nearly-new machines from a museum of nearly-dead ones. Keeping cost and accumulated depreciation separate keeps both facts: carrying amount (book value) = cost − accumulated depreciation = 2,40,000 − 4,000 = ₹2,36,000.
Analysts use that gross detail all the time. The ratio of accumulated depreciation to gross cost is a quick age-of-assets test. And in Phase 2 you'll see that a company whose capex keeps running far below depreciation is slowly liquidating itself. Be careful about what accumulated depreciation isn't. It isn't cash, and it isn't a fund saved up for replacement. It's just the used-up portion of an old purchase price. Depreciation expense is April's slice, an income-statement flow that closing resets. Accumulated depreciation is every slice ever taken, a balance-sheet stock that grows until the asset is sold or scrapped.
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Allowance for doubtful accounts and other contra accounts
Allowance for Doubtful Accounts works the same way. It's a contra-asset to Accounts Receivable, also credit-normal. Gross receivables of ₹1,20,000 is what customers legally owe and what the collections team chases. The allowance of ₹2,400 is what experience says will die. Net realizable receivables of ₹1,17,600 is what the balance sheet claims as an asset. When a specific customer does default, the write-off uses up the allowance (Dr Allowance / Cr Accounts Receivable). No new expense is booked at that point, because the expense was recognized when the risk was estimated. A rising allowance ratio signals customers getting worse, or a management that under-provided before. When you look at Indian banks later in the programme, you'll meet this account's giant cousin, loan-loss provisions.
You'll meet other contras later, and the logic is the same every time. There's sales returns and allowances, a contra-revenue. There's treasury stock, a contra-equity in the equity module ahead, and discounts and premiums on debt in the liabilities and financing module. Whenever netting would hide something a reader needs, accounting splits the account in two.