[AtlUmpSteve]

Here is a similar/related Approved Ruling from Ask Dee.

A.R.8.3.4^2: The Flex bats in a spot in the batting order other than the DP’s spot and before the next pitch, the defense appeals to the umpire for a batting out-of-order violation. They are correct in that the previous batter is not the correct batter but she is an illegal player, not a legal player guilty of batting out-of-order. Should the umpire act on the appealed batting out-of-order or enforce the penalty for illegal player?
RULING: A coach making an appeal must reasonably identify the rule violation in question. While not required to identify the rule by number or page, the player and terminology for the rule in question have to be correctly communicated to the umpire. To avoid miscommunication, the plate umpire should show the defensive coach the lineup card and have him/her walk through the complaint giving the coach every opportunity to see the mistake and better articulate the appeal. In the unlikely event that the coach still only appeals batting out-of-order, the plate umpire shall enforce the effect for the appealed batting out-of-order. (See also Rule 11.10.)

The point being that we cannot assume the FLEX batting was intended as an unreported sub for the DP and batting-out-of-order. To make that assumption basically makes the rule declaring the violation (FLEX entering on offense for someone other than DP) an illegal player an impossible occurrence. Now, if the coach reported FLEX playing offense for DP, and THEN the FLEX batted-out-of-order, sure, that's all it is.

In the case of the OP, if they are appealing an illegal player, they have one; and Andy's cite is spot on. If they are appealing batting-out-of-order, then we need more specific information regarding the time of appeal, etc.