Are values in STEP part 21 (ISO 10303-21) CAD files freely embeddable in entities?

Question

Are values in entities freely embeddable in an ISO 10303-21 formatted file?

In all the examples I've seen that were generated from CAD programs, length_measure is embedded in uncertainty_measure_with_unit, like this:

#83 = UNCERTAINTY_MEASURE_WITH_UNIT (LENGTH_MEASURE( 0.0001 ), #86, 'distance_accuracy_value', 'NONE');

Is this required? Could this be changed to, for example:

#83 = UNCERTAINTY_MEASURE_WITH_UNIT (#10000, #86, 'distance_accuracy_value', 'NONE');
#10000 = LENGTH_MEASURE( 0.0001 );

Conversely, we see something like this:

#91 = APPROVAL ( #92, 'UNSPECIFIED' ) ;
#92 = APPROVAL_STATUS ( 'not_yet_approved' ) ;

Assuming #92 is not used by any other entity, could this be changed to:

#91 = APPROVAL ( APPROVAL_STATUS ( 'not_yet_approved' ), 'UNSPECIFIED' ) ;

The reason I ask is, I don't have access to ISO 10303, since it is locked away behind many hundreds of dollars of paywall, and none of the sites dealing with STEP talk about embedding values.

Answer 1

I assume here that you use ISO 10303-203. The EXPRESS file modeling the AP203 is available through the SCL project here. This is not the official ISO resource (stepmod is).

1. LENGTH_MEASURE is defined as a type in the EXPRESS file so it cannot be specified as an entity declaration.

   TYPE length_measure = REAL; END_TYPE;

2. Each entity shall be expressed on a separate line even if it is not used by any other entity.

You can get more information about STEP physical file on the Depuis website or directly from the ISO 10303-21 standard page. This wikipedia page can also help.