Might be a mask for a strained layer (like SiGe or
some SOI)? I have not heard of anyone "straining"
(deforming) a mask. Although I guess with all the
kooky optics in modern leading edge litho nothing
can be ruled out.
Find your process PDK / docs and the layertable
ought to be there. Follow keywords through the
other design docs (process construction stuff is
often in the front matter of modeling or design
training manuals).