Add a page explaining script purposes (#6403)

Author: zliu41

doc/docusaurus/docs/working-with-scripts/script-purposes.md

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+---
+sidebar_position: 2
+---
+
+# Script Purposes
+
+One of the arguments every Plutus script receives is the script context, containing information about the transaction the script is validating.
+One of the fields of script context is the script purpose.
+Since a transaction may do multiple things, each of which is validated by a separate  script, the script purpose informs a script what exactly it is supposed to validate.
+
+Plutus V1 and V2 share the same [`ScriptPurpose`](https://plutus.cardano.intersectmbo.org/haddock/master/plutus-ledger-api/PlutusLedgerApi-V1-Contexts.html#t:ScriptPurpose) type with four variants: spending, minting, rewarding and certifying.
+Plutus V3's [`ScriptPurpose`](https://plutus.cardano.intersectmbo.org/haddock/master/plutus-ledger-api/PlutusLedgerApi-V3-Contexts.html#t:ScriptPurpose) has two additional variants: voting and proposing.
+Next we go over and explain all these different script purposes.
+
+## Spending
+
+A spending script validates the spending of a UTXO.
+A UTXO can have either a public key address or a script address.
+To spend a UTXO with a script address, the script whose hash matches the script address must be included in the transaction (either directly, or as a reference script), and is executed to validate it.
+
+The script can make the decision based on the datum attached in the UTXO being spent, if any (datum is mandatory for Plutus V1 and V2, i.e., a UTXO with a Plutus V1 or V2 script address but without a datum cannot be spent; datum is optional for Plutus V3), the redeemer included in the transaction, as well as other fields of the transaction.
+
+The `Spending` constructor of `ScriptPurpose` includes a `TxOutRef` field that references the UTXO the script is validating, which is also one of the UTXOs the transaction attempts to spend.
+
+## Minting
+
+Minting scripts, sometimes also referred to as minting policies, are used to approve or reject minting of new assets.
+
+In Cardano, we can uniquely identify an asset by its `CurrencySymbol` and `TokenName`.
+If a transaction attempts to mint some new assets, then for each unique `CurrencySymbol` in the new assets, the script whose hash matches the `CurrencySymbol` must be included in the transaction, and is executed to validate the minting of that `CurrencySymbol`.
+This is the reason why the `Minting` constructor of `ScriptPurpose` contains a `CurrencySymbol` field.
+
+A minting script should pay attention to the fact that the transaction may attempt to mint multiple assets with the same `CurrencySymbol` but different `TokenNames`.
+In general all these assets with the same `CurrencySymbol` should be checked.
+
+## Rewarding
+
+As previously stated, a UTXO's address can be either a public key address or a script address.
+Both kinds of addresses can optionally have a staking credential.
+A UTXO may contain Ada, and the Ada it contains can be delegated to an SPO to earn staking rewards.
+Staking rewards are deposited into a reward account[^1] corresponding to the staking credential.
+
+A staking credential can contain either a public key hash or a script hash.
+To withdraw rewards from a reward account corresponding to a staking credential that contains a script hash, the script with that particular hash must be included in the transaction, and is executed to validate the withdrawal.
+
+The script might set conditions on reward distribution, such as ensuring that any withdrawal are deposited into one of the designated addresses.
+
+## Certifying
+
+A certifying script can validate a number of certificate-related transactions, such as: (1) registering a staking credential, and in doing so, creating a reward account associated with the staking credential; (2) de-registering a staking credential, and in doing so, terminating the reward account; (3) delegating a staking credential to a particular delegatee.
+
+In all these cases, if the staking credential in question contains a script hash (as opposed to a public key hash), the script with that hash must be included in the transaction, and is executed to validate the action.
+
+Such a script may, for instance, check that certain signatures be provided for registration, de-registration and delegation, or that the delegatee must be among the allowed delegatees.
+
+## Voting
+
+A voting script validates votes cast by a Delegated Representative (DRep) or a constitutional committee member (CCM) in a transaction.
+A DRep or a CCM can be associated with a script hash.
+If a transaction contains one or more votes from a DRep or a constitution committee member associated with a script hash, the script with that hash must be included in the transaction, and is executed to approve or reject the vote.
+
+## Proposing
+
+A proposing script, also known as constitution script or guardrail script, validates two kinds of [governance actions](https://plutus.cardano.intersectmbo.org/haddock/master/plutus-ledger-api/PlutusLedgerApi-V3-Contexts.html#t:ScriptContext): `ParameterChange` and `TreasuryWithdrawals`.
+
+There is a key distinction between proposing scripts and other kinds of scripts: proposing scripts are not written by regular users.
+At any given point in time, there is exactly one active proposing script being used by the entire chain.
+This proposing script must be included in transactions that propose `ParameterChange` or `TreasuryWithdrawals`.
+The ledger enforces that no other proposing script is accepted.
+The proposing script is updated only when there is a change to the constitution, via the `NewConstitution` governance action.
+
+Note that what the proposing script decides is whether the _proposal_ of a governance action is allowed to go through, rather than whether the governance action will be enacted.
+After a proposal goes through, it will need to meet the appropriate criteria (such as gathering enough votes from consitution committee members, DReps and/or SPOs) in order to be enacted.
+
+---
+
+[^1]: Reward accounts are distinct from UTXOs, and are more akin to accounts in account-based blockchains.
+They are used for accumulating staking rewards, rather than using UTXOs, in order to avoid creating a large number of UTXOs with tiny values.
+Reward accounts are special on Cardano and cannot be created arbitrarily.
+Each reward account is associated with a staking credential, and is created automatically when the staking credential is registered.