Junior assignments for JGP are at discretion of the skaters country. So for example, Russian fedration decides about which Russian skaters will fill out the spots that are allocated for Russia. Russian federation has 14 spots for ladies and can send there any junior eligible skaters, it can give 7 skaters two slots each, or 14 skaters one slot each (which would be pretty stupid because no Russian lady would get to JGPF), or any combination of ladies up to the 14 spots. The Russian federation can allocate the slots and then swap it any way they want to.
Senior GP is different. The skaters are invited by the host country, based on their past results. So if Russian lady (e.g. Tsurskaya) withdrew, it is not up to Russian federation to choose replacement. The host country would be choosing replacement, and it wouldn't have to be another Russian skater they would choose. GP events are also impossible to swap, so if Tsurskaya was injured, she can't swap with another Russian lady their eventsfor any later event.
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