A child that is in the care of the local authority, or was in their care but has now been adopted.
It does not (unless it has changed) include children adopted from abroad.
It definitely does not include step-parent adoption.
A child informally being cared for by relatives without any SS involvement won't count. A child 'placed' with relatives via SS as kinship carers under eg SGO should count, I think.
I don't think (but could be wrong) it includes children who had been looked after children but have been returned to their birth parents.