We actually independently started using the exact same term too! We have parallel life experiences that give us multiple bodies as well as walk-ins, so it's very useful for us to distinguish between "formed related to this body" and "formed in the headspace".

It's a grey area a lot of the time because some of us are technically walk-ins but got modified in the headspace, and with a few parallel lives we've had headmates go between them freely since the body was a child and with those we straight up can't tell which body some of them formed in.