In order to have existing steel not included in the numbering, the user has to go into the advance properties and change the behaviour, prior to running the numbering.
However, the properties tab doesn't have assembly numbering properties available to be checked/unchecked until after the numbering, as they are assigned to the main part during the numbering process. This means having to take multiple steps to run numbering, go through the existing steel parts/assemblies resetting the behaviour to not be included, and rerunning the numbering again. (On a recent project I had a large amount of existing steel and in trying to be proactive I made sure to set the model roles and uncheck the numbering behaviour as a created the elements. As only the fields for single parts were available at the time, I had thought that that the numbering command would recognize to not assign assembly/main part numbering, however it did not.)
In order to simply this process, could an option to exclude existing steel be added to the numbering command, in the same way that you can already exclude bolts, anchors, concrete, timber, etc? The "exclude existing" option could be based on the model role "existing", and used to exclude the items for numbering both single parts and assemblies/main parts in 1 step.