Coal and power contractors
Mining and thermal operations bring contractors working to audited terms with purchase orders and gate registers. What clears at their end is an invoice whose order reference, HSN code and line-wise tax all survive a check against the purchase order they raised, which means that reference has to be captured while the bill is written rather than pencilled on afterwards.
Gate records that must match
Material entering a plant or colliery compound is logged at both ends and reconciled at month end. A challan carrying the date, vehicle number, quantity and receiver makes that short, and over the e-way bill threshold the figures are already on the record.
Four buyers, four payment calendars
A colliery contractor, a power station contractor, a cement works contractor and an ordinary district builder are all on different release cycles, so one outstanding total tells you very little about when money actually arrives. Ageing split party by party is what shows the shape of it, and a credit limit set per contractor stops three slow cycles overlapping into a month the shop cannot fund.