The burr got its name from the fact that it is a six-piece burr constructed from 3x3x6 sized pieces of which only an outline of that piece form is used. These pieces assemble into a quite symmetrical form which has one 3-fold symmetry axis. If one only allows pieces which will not leave outside holes in the structure, then there are a total of 11 interesting pieces (see below).
There are 13 possible pieces when outside holes are prohibited. Pieces 12 and 13 are not interesting however since they do not offer much interlock. This leaves only 11 usable pieces.
Pieces | W | Asm | Sol| Level 22.214.171.124.3.3 | 42 | 24 | 1 | 5.1.1 The Symmetrical One (Pictures) 126.96.36.199.5.8 | 41 | 86 | 1 | 15.1 The Ultimate Level 188.8.131.52.8.9 | 40 | 96 | 12 | 6.1-7.5 (Karin's Original Outline Burr)
Here you can see Karin and Jürg at IPP21 in Tokyo, with their puzzles on the table.