My undestanding:

1.For Sketches 1.1 through 1.3, and Sketch 3.1 through 5.1:
Use SSI = SIF

2.For Sketches 2.1 through 2.6:
Use SSI = min of (1)(2)(3)
(1) = 0.75 x applicable_SIF
(2) = (t/T) x sqrt(applicable_SIF) when t/T>1
(3) = sqrt(appplicable_SIF) when t/T<=1

The statement of General Note (d) of B31J should be reviewed again in my opinion

You can check the latest online help document version 13 for clearly understanding, section "Sustained Stress Index or Moment Factors (SSI)", it is saying:

"For tees and branch connections, CAESAR II uses a special B31J scaled value of SSI that depends on the ratio of branch to run thicknesses" --> means SSI=min(1)(2)(3) for sketches 2.1 through 2.6

"For bends, reducers, welds, and joints, the B31J applies the full SIF values for the SSI" --> means SSI=SIF for sketches 1.1 though 1.3, and 3.1 through 5.1