So plaster, carpet and wallpaper are aesthetic items to make your base look nicer. To use plaster, first take your bucket in your inventory and hold alt and right click it to separate it into another bucket. You should be able to get 8 applications out of a full bucket. Carpet and wallpaper rolls also come in stacks of 8, but it is clearer how to separate them. Next, go to the inventory of a roof or floor and place the plaster bucket in one of the (4) current slots that say plaster. Depending on the slot you put it in, it will have a different appearance. Now look up at your roof and see the difference. Note, if you put the plaster on a floor, it changes the look of the ceiling below, meaning plaster on floors should only be used on floors above the first level. Carpet works the same way, it can only be applied to floors and comes in 8 different looks. Wallpaper also works the same way, only it can only be applied to the inside of a BBP wall. Hopefully we will see an exterior slot for walls in the future.
I hope that helped.

THANK YOU!!! I really appreciate your reply ShadowDBA! It's typically difficult to find someone who is willing to take the time to write a comprehensive, grammatically correct explanation, so I hope a lot of other folks will benefit from your how-to piece as I did. I have two accounts which I use solely to play DayZ. I deleted my primary email after getting a private email address, and I locked myself out of my old Steam account, on which I had racked up 4000+ hours on DayZ. Even with the hours I've racked up in DayZ, the BBP mod is still fairly new to me. I just recently started using BBP, and I've finally figured out how to use it efficiently and properly, but I couldn't figure out how the plaster fit into the picture. It makes sense now that you've explained it, LOL. I have carpet and wallpaper in my base, but plaster was literally non-existent on the server I played on. I was told by people that "you have to apply it to your walls", and other ridiculous things, but it makes sense that I would have to drag the plaster bucket to one of the four slots in the materials tab, just like carpet and wallpaper. I guess I have four different choices of style and color with the plaster tab too! After searching the internet for hours with no luck, I finally learned the last piece of the BBP puzzle thanks to your reply. You helped me immensely!

You know, if you didn't put in AAAAALLLLLLLL THE EMPTY SPACE below you question, people might also find the answer, you buffoon.