[packages/doc] define and document package rules

Central place for putting package-related info. This document or parts
of it can also be easily ported to other places if deemed appropriate.

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diff --git a/doc/policy/README.md b/doc/policy/README.md
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+# Transaction Relay Policy
+
+Policy is a set of validation rules, in addition to consensus, enforced for unconfirmed
+transactions.
+
+This documentation is not an exhaustive list of all policy rules.
+
+- [Packages](packages.md)
+
+
diff --git a/doc/policy/packages.md b/doc/policy/packages.md
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+# Package Mempool Accept
+
+## Definitions
+
+A **package** is an ordered list of transactions, representable by a connected Directed Acyclic
+Graph (a directed edge exists between a transaction that spends the output of another transaction).
+
+For every transaction `t` in a **topologically sorted** package, if any of its parents are present
+in the package, they appear somewhere in the list before `t`.
+
+A **child-with-unconfirmed-parents** package is a topologically sorted package that consists of
+exactly one child and all of its unconfirmed parents (no other transactions may be present).
+The last transaction in the package is the child, and its package can be canonically defined based
+on the current state: each of its inputs must be available in the UTXO set as of the current chain
+tip or some preceding transaction in the package.
+
+## Package Mempool Acceptance Rules
+
+The following rules are enforced for all packages:
+
+* Packages cannot exceed `MAX_PACKAGE_COUNT=25` count and `MAX_PACKAGE_SIZE=101KvB` total size
+   (#20833)
+
+   - *Rationale*: This is already enforced as mempool ancestor/descendant limits. If
+     transactions in a package are all related, exceeding this limit would mean that the package
+     can either be split up or it wouldn't pass individual mempool policy.
+
+   - Note that, if these mempool limits change, package limits should be reconsidered. Users may
+     also configure their mempool limits differently.
+
+* Packages must be topologically sorted. (#20833)
+
+* Packages cannot have conflicting transactions, i.e. no two transactions in a package can spend
+   the same inputs. Packages cannot have duplicate transactions. (#20833)
+
+* No transaction in a package can conflict with a mempool transaction. BIP125 Replace By Fee is
+  currently disabled for packages. (#20833)
+
+   - Package RBF may be enabled in the future.
+
+* When packages are evaluated against ancestor/descendant limits, the union of all transactions'
+  descendants and ancestors is considered. (#21800)
+
+   - *Rationale*: This is essentially a "worst case" heuristic intended for packages that are
+     heavily connected, i.e. some transaction in the package is the ancestor or descendant of all
+     the other transactions.
+
+The following rules are only enforced for packages to be submitted to the mempool (not enforced for
+test accepts):
+
+* Packages must be child-with-unconfirmed-parents packages. This also means packages must contain at
+  least 2 transactions. (#22674)
+
+   - *Rationale*: This allows for fee-bumping by CPFP. Allowing multiple parents makes it possible
+     to fee-bump a batch of transactions. Restricting packages to a defined topology is easier to
+     reason about and simplifies the validation logic greatly.
+
+   - Warning: Batched fee-bumping may be unsafe for some use cases. Users and application developers
+     should take caution if utilizing multi-parent packages.