Weld fatigue performance - bimodal - EXC2 and EXC3

This is as I understand the current situation,
speaking as both

• an engineer and metallurgist
• a commercial welder of structural steels

The axis labels are deliberately a bit vague, but could mean:

• Weld service quality <-> fatigue endurance of the welded structure?
• Weld balance of conditions <-> weld pool fluidity?

[PDF vector-graphic of same image - usually clearer / higher graphic display quality - you are likely to want to study this PDF image]

(R. Smith, 06Jun2021)