Millwall fan here…..from last season, lower tier tickets are now sold first with additional tickets then made available thereafter (in the upper tier). The upper tier is steeper that the opposite stand, and what with persistent standing (fairly normal for most away ends), the safety committee has imposed these restrictions….also, the police have the final say in these arrangements.
Where away clubs do not take up their allocation, Millwall have placed home fans (local community initiatives i.e. schools, kids clubs) in the away end - in a different tier I might add….think this only happened on a few occasions towards the end of last season.