Answer to this question comes from a combination of rules dealing with substitutions and when a substitute becomes a player of record. A substitute becomes a player of record when beckoned by the referee (or when the clock is stopped by the referee during the last 5 minutes of the game and the leading team substitutes) per A.R. 3.7.1.c. The rules do not require that the player identify who is being replaced, only that the player leaving the field leave near the half-way line on the side of the benches (rule 3.7.1). There are many occasions where the coach changes his / her mind before the player leaves as to how will be replaced in a substitution situation. To help better understand this answer, imagine in this situation A1 and A12 get into a fight with B1 and B2, both players of record (instead of each other). It is fair that both teams should play with 9 players.
