I'll add my few cents
I like to keep things logical and simple. So the way I see things is that there are three major styles of casting a double-handed rod with the use of an anchor (non overhead casting). I deliberately refrain from using the term spey casting as some purists would say underhand casting is not spey casting. The three styles are:
1. Underhand casting (originated by Göran Andersson).
- shorter/fast-action rods
- short heads
- short casting stroke
- long leaders
- use of bottom hand alone to sweep and to propel the line into the forward cast
- maintaining high rod tip position from the lift into the forward cast
- slow and relaxed hand movements from the lift into the firing position
- short distance between top and bottom hands on the grip
- high line speed
- airborne anchors
BTW underhand style has its use with both SH and DH
2. Traditional spey casting
- long/slower rods
- long lines
- long casting stroke
- use of top hand to sweep and to propel the line into the forward cast
- long distance between top and bottom hands on the grip (top hand closer to winding check than to the reel seat)
3. Modern spey casting
Anything between 1 and 2. Including Scandinavian casting and Skagit Casting. With the reservation that Scandinavian casting employs bottom hand as a dominant hand to propel the line into the forward cast.
Those are the simplest definitions I can come up with according to my understanding of the styles.
I can cast Skagit heads in Scandinavian style. I can cast mid and long belly lines in Scandinavian style. I can cast Scandi heads in traditional style. It's too much fun too keep it in strict boundaries.