Given any two transactions Tx0 and Tx1 where Tx1 spends an output of Tx0, Tx0 is a parent of Tx1 and Tx1 is a child of Tx0.
A transaction's ancestors include, recursively, its parents, the parents of its parents, etc. A transaction's descendants include, recursively, its children, the children of its children, etc.
A mempool entry's ancestor count is the total number of in-mempool (unconfirmed) transactions in its ancestor set, including itself. A mempool entry's descendant count is the total number of in-mempool (unconfirmed) transactions in its descendant set, including itself.
A mempool entry's ancestor size is the aggregated size of in-mempool (unconfirmed) transactions in its ancestor set, including itself. A mempool entry's descendant size is the aggregated size of in-mempool (unconfirmed) transactions in its descendant set, including itself.
Transactions submitted to the mempool must not exceed the ancestor and descendant limits (aka
mempool package limits) set by the node (see -limitancestorcount, -limitancestorsize,
-limitdescendantcount, -limitdescendantsize).
CPFP Carve Out if a transaction candidate for submission to the mempool would cause some mempool entry to exceed its descendant limits, an exemption is made if all of the following conditions are met:
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The candidate transaction is no more than 10,000 bytes.
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The candidate transaction has an ancestor count of 2 (itself and exactly 1 ancestor).
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The in-mempool transaction's descendant count, including the candidate transaction, would only exceed the limit by 1.
Rationale: this rule was introduced to prevent pinning by domination of a transaction's descendant limits in two-party contract protocols such as LN. Also see the mailing list post.
This rule was introduced in Bitcoin Core PR #15681 backported to Dash Core in PR #4572