Might be wrong (and I hope I am!) but it seems to me like they're purposely saying they want full capacity for any members' sales.
I'm pretty sure what they're getting at.
STHs will take priority (which is generally fair imo). In an ideal world they'd do it on a tiered basis dependent on capacity.
1. STHs + away PL credits (different tiers of 19, 13, 8, 5 or something)
2. STHs with no away PL credits
3. Members with 19 home + away PL credits (different tiers of 19, 13, 8, 5 or something)
4. Members with just 19 home
5. Everyone else is unlikely to have a sniff unless they allow transfers / there's a vaccine.