When using the rule type 'Modify Price of a Unmatched Bet' the Parameter area will allow you to specify the new odds for an unmatched bet, which bet you want to reprice and which bets to search/filter.
The 'Place at' options let you specify where you want the unmatched bet to be repriced in the market, using either fixed odds or odds in relation to the current market price. All the options available are the same as when initially placing the bet and explained in full on the 'Place at (Parameters)' page of this user guide.
The next option allows you to specify which unmatched bet you want to move
The Selection's oldest unmatched bet - Will move the oldest unmatched bet you have on the market (i.e., the one that was placed earliest)
The Selection's oldest unmatched bet at the servant context price - Will move the oldest unmatched bet you have at the servant context price (i.e., the one that was placed earliest at that price)
The Selection's most recent unmatched bet - Will move the most recent unmatched bet you have on the market
The Selection's most recent unmatched bet at the servant context price - Will move the most recent unmatched bet you have at the servant context price
The bet reference held in a Stored Value - Will move the unmatched bet matching a bet reference ID held in a stored value. When this option is used you will be given additional options to specify the name of the Stored Value and if its for the Selection, Market, Event or Bet Angel
The final option you have is to filter your search for unmatched bets on one side of the book or the other
Search Back bets only - Looks only at the unmatched back bets
Search Lay bets only - Looks only at the unmatched lay bets
Search Back and Lay bets - Looks at unmatched bets on both sides of the book
NB: once a bet's price is changed, betfair will then issue a new reference for that bet
