1. Substitutes and runners
(a) If the umpires are satisfied that a player has been injured or become ill after the nomination of the players, they shall allow that player to have
(i) a substitute acting instead of him in the field.
(ii) a runner when batting.
Any injury or illness that occurs at any time after the nomination of the players until the conclusion of the match shall be allowable, irrespective of whether play is in progress or not.
(b) The umpires shall have discretion, for other wholly acceptable reasons, to allow a substitute for a fielder, or a runner for a batsman, at the start of the match or at any subsequent time.
(c) A player wishing to change his shirt, boots, etc. must leave the field to do so. No substitute shall be allowed for him.
2. Objection to substitutes
The opposing captain shall have no right of objection to any player acting as a substitute on the field, nor as to where the substitute shall field. However, no substitute shall act as wicket-keeper. See 3 below.
3. Restrictions on the role of substitutes
A substitute shall not be allowed to bat or bowl nor to act as wicket-keeper or as captain on the field of play.
There is no law regarding who your substitute(s) must be - hence the case of the Danish lad, Fredrik Klokker, fielding for England at the Lord's Test.