I'm still confused on the use case.
A YSA that's no longer living at home needs to have their records where they are living/attending.
A YSA that's at home but attending a YSA ward would either have their records moved to that ward, or at minimum, an out-of-unit record. However, I don't think out-of-unit members can participate in Ministering.
The only time this would seem to be an issue is in a home ward when the YSA is a "child" in the household. There's nothing written in stone that an adult must be listed as a child if they live at the same address as the parents. They can be spun off into their own household. That's up to the bishop. The desire to give them their own ministers from the EQ would drive that decision.
Bottom line, how does this affect a YSA ward? Assuming the membership record is in the ward, there's no reason the number couldn't be the same as the personal number. Perhaps an out-of-unit member doesn't allow editing of contact information, but that raises a bigger question of if the records should be transferred. Either way, only one ward misters to them. And it's the one holding the actual record.
I'm missing something about this puzzle.
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