As outlined in the General Match Requirements, for Youth football matches (U12 to U16), each team is required to provide at least one Boundary Umpire for each game. This person is required to act according to the rules of the game (as outlined below).
The following information has been taken from the AFLs "Laws of Australian Football 2012"
8. Umpires: Duties and Instructions
8.1 Appointment and Duties of Umpires
Unless Law 8.1.2 applies, a Controlling Body shall appoint for each Match:
(a) a field Umpire or Umpires;
(b) two or more boundary Umpires; and
(c) two goal Umpires.
8.1.2 No Appointment of Boundary/Goal Umpires
(a) Where it is impractical to appoint or the Controlling Body has not appointed a boundary or goal Umpire to officiate at a Match, the field Umpire(s) may, in consultation with each Team, appoint persons before the commencement of the Match to officiate in those roles.
(b) Unless otherwise determined by a Controlling Body, a field Umpire may overrule the decision of a boundary or goal Umpire who was appointed under Law 8.1.2 (a).
(c) When no boundary or goal Umpire has been appointed under Laws 8.1.1 or 8.1.2 (a), the duties normally undertaken by a boundary and goal Umpire shall be undertaken by the field Umpire.
8.1.3 Replacement of Field Umpire
A Controlling Body shall adopt rules which prescribe the replacement of an Umpire who, by reason of injury or otherwise, becomes incapable of officiating either before orduring a Match.
8.1.4 Non-attendance of Field Umpire
Where a Controlling Body has not appointed a field Umpire or for any reason a field Umpire is unavailable or does not attend to officiate at a Match, the competing Teams may elect to play the Match and upon doing so, may appoint an Umpire(s) (inclusive of field, boundary and goal Umpires), to officiate at the Match. The relevant Controlling Body may determine in its absolute discretion whether to recognise the result of such Match.
8.2 Duties of Umpires
8.2.1 Field Umpires
The field Umpires shall officiate and have full control of a Match.
8.2.2 Boundary Umpires
Unless determined otherwise by the relevant Controlling Body, the duties of a boundary Umpire include without limitation:
(i) judging whether a football is Out of Bounds or Out of Bounds on the Full and signalling to the field Umpire when that has occurred;
(ii) throwing the football back into play if it has gone Out of Bounds, when directed to do so by a field Umpire;
(iii) determining whether a Player has incorrectly entered the Centre Square in contravention of
Law 11.3.4 including notifying the field Umpire of Centre Square infringements;
(iv) bringing the football back to the Centre Square after a Goal has been scored; and
(v) reporting a Player or Official who commits a Reportable Offence.
(vi) Assisting goal umpires to determine a score.
(b) Bringing Football into Play
When the boundary Umpire signals that the football is Out of Bounds, the field Umpire shall immediately sound a whistle to indicate that the football is out of play. When directed by the field Umpire, the boundary Umpire shall bring the football into play by throwing the football over their head towards the centre of the playing Surface. The field Umpire may recall any throw in by the boundary Umpire that in their opinion does not allow any opportunity for the football to be contested.
8.2.3 Out of Bounds
Where the field Umpire has failed to notice a boundary Umpire’s signal that the football has gone Out of Bounds or Out of Bounds on the Full, the following shall apply:
(a) the boundary Umpire shall continue signaling until they are noticed by the field Umpire;
(b) upon noticing the boundary Umpire’s signal, the field Umpire shall stop play and direct that the football be taken to where it crossed the Boundary Line, at which point the football shall be thrown in by the boundary Umpire or Kicked back into play, as the case may be. This provision does not apply if a Free Kick or Fifty-Metre Penalty is awarded before the field Umpire notices the boundary Umpire’s signal;
(c) this Law applies even if a Goal or Behind is scored before the field Umpire notices the boundary Umpire’s signal. In such instances, the field Umpire shall direct the goal Umpire to annul the score.
Reference Source: Laws of Australian Football 2012