I'll carry on sticking my neck out :-}

In procedures we write, a single optional sub-step would be included within the step it depends on:


Code:
2) Uncouple thermocouple union at valve. If electronic ignition, remove flame rod and ignition leads.
With multiple sub-steps, step 2 would be a summary instruction followed by a, b, c. I don't remember ever coming across the 2a … b style you're aiming for.