See below, taken from the Competition Rules (they can be downloaded here -
http://files.pitchero.com/clubs/15847/SouthYorkshireTrophy-CompetitionRules.pdf):
2. ELIGIBILITY OF PLAYERS2.1 Players shall be restricted to paid up playing members of clubs within Districts 8,9,10 and 11 of the Yorkshire RFU. Guest players from one Club may not play for another.
2.2 All players from Clubs whose 1st team play in Yorkshire leagues 3, 4 & 5 are eligible.
2.3 Players from Clubs whose 1st team play in Yorkshire leagues 1, 2 and above, shall only be selected from a club's 2nd XV and below.
This may include any player who has not started five or more times for his Club’s 1st XV in the current season in any League, Cup or Friendly game, prior to the start of that season’s competition.