.neck files control how far a character's head turns. Some posers allow to set that a character's head follows the position of the camera.
.neck units are similar to .eyes units and accompany xx units for character's heads. They have the same name and are located in the same .pp file.
The contents can be view as text with the NeckTool. This command line tool can also be used to convert such text output back to a .neck file. Here an example for decoding:
CMD> NeckTool.exe /decode cw_body_00_00.neck
unknown: x00 x01
id: x00 x00
values: i0 0.8726646 0.6981317 i250 i0 1 0.6981317 0.5235988 i250 i0 i250 i0 -0.8726646 -0.6981317
id: x02 x00
values: i7 0.6981317 0.4363323 i250 i0 0.6 1.745329 0 i100 i0 i1000 i500 -0.6981317 -0.4363323
id: x04 x00
values: i5 0.6981317 0.08726646 i250 i0 1 0.2617994 0.3490658 i1000 i800 i500 i0 -0.6981317 -0.08726646
id: x05 x00
values: i7 0.1745329 0.1745329 i250 i0 0.5 0.2617994 0.8726646 i1000 i0 i500 i0 -0.1745329 -0.1745329