Difference between "fixed" & "floated"

Hi everyone,

after some troubles with meshing my 3D CAD model, I handle it now 8-).
However I don't understand some details of the 3D CFD TETRAMESH PANEL, after done tutorials.

It's clear to choose between the options w/ & w/o BL, but what is the difference of FIXED and FLOATED BLs?

Thanks for your help  !!!