So before I look in ASTM for the correct terminology, what is the product called that you use beneath a brick or tile setting bed and on top of a concrete slab that creates a slip plane allowing differential movement. I have seen all of these terms used before. Is there a preferred term to use?
Would think best term is the word that best describes its function,“slip sheet”
It has no waterproofing function nor any structural function-- it is there to permit the movement or slippage!
The new 2011 TCNA handbook has included several new sections. One section is the membrane selection guide. It list 5 different membranes for use with tile or stone.
Cleavage membrane is a thin layer of material with a tile assembly that is loose laid (floating). Cleavage membranes are incorporated below the mortar setting bed in a thick bed tile installation. When a cleavage membrane is incorporated, the setting bed is required to be reinforced with lath or wire.
Crack Isolation Membranes (ANSI A118.12) for thin-bed ceramic, glass, and stone installations act to isolate the tile from minor in plane substrate cracking. Membranes covered by this definition are bonded to a variety of manufacture approved substrates covered by ANSI specifications.
I’d ask my BIA guy. 
Crack isolation is exactly what it says. In theory it should prevent cracks in the concrete slab from propagating thru the ceramic tile. It can be used with thick or thick bed setting methods.
A slip sheet (cleavage membrane) divorces the substrate, usually a concrete slab, from the ceramic tile. It allows independent movement of both. It is used for suspended slabs which, on the West Coast, tend to be on steel deck and deflect more than CIP concrete structures.
A Crack isolation can also serve as a slip sheet, but the reverse is not true.
The Tile Council Of America (TCNA) & ANSI use the term Cleavage Membrane
Cleavage Membrane per ANSI #15 roofing felt or 4 mil polyethylene film, used under the mortar setting bed in a thick bed installation.
ANSI A118.12 Crack Isolation Membranes: this standard applies to trowel applied, liquid and sheet membranes
Slip sheet: any material the tile contractor may choose.
The other unrelated application I can think of where these terms bounce around is for the thin sheet material you place between a metal roof system or similar and a substrate it might stick to, often covered in a peal-and-stick sheet waterproofing.