The term "spinal" is used in at least two ways.
In Pick&Mix, a spinal level is one of the levels 1, 3, and 5. Typically in developing scoring statements, one focusses on the spinal levels first and then derives scoring statements for levels 2 and 4. (By the nature of level 6, this cannot be done by interpolation or even by extrapolation.)
The process is described in a report on criteria creation.