DESCRIPTION:

Many renderers in Eccet can visualize different parts of the
volume in different colors, if the class field is present.

The field has a type of (see fieldtype) U8, i.e. an unsigned
byte with an allowed value range of 0-255 and is called "Class".
The values 0-31 are normally reserved for grey values, while
32-255 mark surfaces (see /norm/al and grey for an explanation
of this overloading).

The field can be allocated using VOL_ALLOC_CLASS, if needed.