Inside the larger SketchUp User Guide is:
And here in this category, I created a wikilist of DC Resources:
And I myself use Aerilius’ attribute Inspector extension.
The lenx_nominal is a custom attribute, so has no global relational meaning.
It’s use is how the author of that particular DC decided to relate it via formulae.
lengthunits are something built-in (global default) and I think apply to the len_x, len_y and len_z attributes, (perhaps also X, Y and Z.) (I think they get toggled between cm and inches by clicking the ruler icon in the DC Attributes dialog.)
formulaunits I think are a global default setting, but also can vary per custom attribute, ie my_color_setting_formulaunits would automatically be applied to my_color_setting attribute.
Trial and error. Play with changing attributes using the Attribute Inspector.
Attribute names beginning with an underscore are hidden attributes.