CH02::coin selection: make its own subsection

ch02.asciidoc

@@ -261,9 +261,6 @@ like currency notes, cannot be divided. If you purchase a $5 US dollar
 item in a store but use a $20 dollar bill to pay for the item, you
 expect to receive $15 dollars in change. The same concept applies to
 bitcoin transaction inputs. If you purchased an item that costs 5
-
-Different wallets may use different strategies when aggregating inputs
-to make a payment requested by the user. They might aggregate many small
 bitcoins but only had an input worth 20 bitcoins to use, you would send one
 output of 5 bitcoins to the store owner and one output of 15 bitcoins back
 to yourself as change (not counting your transaction fee).
@@ -280,6 +277,13 @@ otherwise look identical, preventing any third party from determining
 which outputs are change and which are payments.  However, for
 illustration purposes, we've added shading to the change outputs in
 <<transaction-chain>>.
+
+==== Coin selection
+
+Different wallets use different strategies when choosing which
+inputs to use to a payment, called _coin selection_.
+
+They might aggregate many small
 inputs, or use one that is equal to or larger than the desired payment.
 Unless the wallet can aggregate inputs in such a way to exactly match
 the desired payment plus transaction fees, the wallet will need to