[Ansteorra] "Period Containers"

[Quill]

Just a lighthearted question here. I've seen this phrase used since someone
handed me my first event flyer, and it only just occurred to me to wonder
about it. What does it mean "period containers" at site that are designated
wet? Is that just a fancy way of saying "no kegs, hide the cans" or should I
invest in an authentic wineskin in the future? :)
/|\ Quill