The result is a List, where each item is the number of the buffer associated with each window in the current tab page. {arg} specifies the number of the tab page to be used. When omitted the current tab page is used. When {arg} is invalid the number zero is returned. To get a list of all buffers in all tabs use this:

let buflist = []
for i in range(tabpagenr('$'))
   call extend(buflist, tabpagebuflist(i + 1))
endfor

Note that a buffer may appear in more than one window.

Can also be used as a method:

GetTabpage()->tabpagebuflist()