The jacket is described as being made of denim and being lined with wool. It has many pockets which is lucky for Adaine as that's rare for woman's clothing. The jacket was previously a robe but the previous owner wanted to spiff it up and give it a new look.
The Jacket and Living Creatures
As soon as Adaine gets the jacket, Fig immediately asks for a rat. The shopkeeper, Madam Silvaine, warns that any living being produced from the jacket will be cursed with a "half-life" and be in immense pain which causes any living creature to immediately try to kill itself. Fig then asks for an Oyster because they don't have a central nervous system and therefore don't have the capacity to feel pain, however, Fabian tells them a paradox. Adaine gives Fig an Oyster in a glass of water that she also takes out from her jacket. Fig names it 'Little Gilear' and keeps it until it's "way past dead" and has made a horrible stinking odor.
Civilization in the Jacket
After leaving The Gilded Coin, Adaine Abernant reaches into the pocket and asks for the last thing the last owner put in there. Adaine produces a note that reads "i'm hiding in the jacket, please don't take me out." Adaine then asks how many people are inside the jacket, she produces a note that reads "mind your own fucking business."
Fig Faeth tries to cast Detect Thoughts on the jacket but ends up getting an overwhelming amount of of thoughts, thousands maybe millions of streams of consciousnesses. Fig becomes overwhelmed and takes ten points of damage.
Adaine's Identify spell then explains "that the creatures taking refuge in the jacket are various refugees, criminals, and hideaways that live in a city at the centre of the extra dimensional space within the jacket. They live complex lives form relationships, nations rise and fall, and they have no way of exiting the jacket. It's more of a thought experiment really and has no ability to interfere with you living your life, just sort of a mind fuck for you to walk around with."
The people living in the jacket seem to be aware of where they are living, as in Episode 16, in order to save the entire realm within the jacket, a man jumps out of the pocket to aid Adaine Abernant in the battle against Kalvaxus.
The jacket is able to produce useful items from any of its pockets when asked. Anything worth more than 10 gold pieces like gold and jewels cannot be produced by the jacket. Any living creature you pull out will live what is known as a "half-life" and will immediately try to kill itself.