Amortisation is the accounting method that spreads a transfer fee across the length of a player's contract instead of charging it all in one year. A fee on a five year deal counts as one fifth of the cost in each of those five years. Long contracts therefore ease the pressure of spending rules.
It explains a pattern that otherwise looks strange: clubs handing long contracts to players who are already established. A longer deal means a smaller annual charge against the accounts, and spending rules are measured annually, so the length of the contract is a financial decision as much as a sporting one.
Governing bodies eventually capped it. UEFA limited how many years a fee can be spread over for the purposes of its own rules, which stopped clubs stretching contracts to unusual lengths purely to flatten the accounting charge.
Amortisation also explains why selling academy players is so useful. A graduate cost nothing to buy, so their book value is close to zero and almost the entire sale price is recorded as profit, while selling a recent signing part-way through their contract can register a loss even at a healthy fee.