The values in the path constraint properties are measured in different units depending on the specific property:

1. Position: This is measured in percentages (%) of the path length. A value of 0% is the start of the path, and 100% is the end of the path.
2. Spacing: This can be measured in units or percentages. When set to "Length", it uses the same units as the path's length. When set to "Percent", it uses percentages of the path length.
3. Rotate Mix and Translate Mix: These are measured in percentages (%) and determine how much the path constraint affects the rotation and translation of the constrained bones.

For more detailed information, you can refer to the Path Constraints - Spine User Guide.

The distances for path constraints are in world coordinates. There is no option to use local coordinates, so instead it's typical to use Percent Spacing.

Nothing in a skeleton is ever in pixels. The units are either world coordinates or bone coordinates (where the origin is the bone location).

Pixels only come into play when you render the skeleton. If you render it so world coordinates are 1:1 with screen pixels, then you could consider world coordinates to be pixels. For example, in the Spine editor if you are at 100% zoom then world coordinates are 1:1 with screen pixels.