In general, position is a nonessential variable when qualifying a welding procedure per ASME Section IX. It is a supplementary essential variable when qualifying for impact toughness.
The essential variables of one welding code or standard are not always the same as another due to differences in the organization’s philosophy. In my opinion, ASME concentrates on the mechanical properties of the weld, not whether the WPS will result in a weld that can be made. Again, in my opinion, AWS considers the mechanical properties and whether or not the weld can be successfully made. I’m sure the philosophy followed by ASME is influenced as a result of being sued in the “early years” by a boiler manufacturer that based their case on their view that ASME was setting themselves up as a monopoly and telling manufactures how to build boilers. ASME’s defense was that they do not tell manufacturers how to build boiler (or anything else).
ASME doesn't tell you how to design or build a boiler, pressure vessel, or piping system. They don't tell you how to make a weld or what filler metal is compatible with which base metals. ASME takes the position that you are competent to do what you are attempting to do. In other words, if you are tasked with writing a WPS based on the PQR, you are expected to know enough about welding to provide reasonable ranges for the voltage, amperage, travel speed, etc. for use by the welder. ASME Section IX or the construction codes are not going to provide much in the way of guidance about writing a usable WPS. It is possible to write a WPS that meets ASME Section IX that will not work in production. For example: Joint design is a nonessential variable per Section IX, therefore you can change the groove angle with a simple edit of the existing WPS. Change the groove angle from 75 degrees to 10 degrees and change the root opening (root gap in some circles) from 1/16 inch to zero with a few strokes of the pen. There is a good chance the welder will no longer be able to achieve complete joint penetration if the base metal is relatively thick. The mechanical properties of the weld deposit may not change, but the joint penetration may well be affected.
AWS welding codes are like cookbooks. They tell you how to make a weld and consider the WPS to be prequalified if you follow their "rules". If you decide you want to do something different, they will allow you to do so if you prove it will produce the required mechanical properties. The AWS has different essential variables because they impose limits on the acceptable ranges of voltage, amperage, groove details, etc. (base on the values used and recorded when qualifying the PQR). The welding parameters of voltage, amps, wire feed speed, etc. are needed by the welder to properly set the welding parameters and to produce welds with predictable mechanical properties (which are influenced by the microstructure, grain size, etc.). If you follow the "direction" provided by the AWS welding codes, you should have a WPS that is reasonable and should produce welds with predictable results.
Best regards - Al