ASME VIII-2 2007 Para 4.3.10.1

If you are farther away than 2.5√Rt from major structural discontinuities (such as nozzles or vessel/equipment supports), then the simple rules apply. Any closer to such discontinuities, you're dealing with more than one thing at a time, and further rules/calculations may apply.

Though there might be Code provisions that seem complex or counterintuitive, and thus confusing, this one is quite straightforward.

Though ASME does not define the Major structural discontinuity, however it defines the Gross Structural Discontinuity.

I assume they are the same.

ASME Section III,Division 1,Subsection NB, Class 1 Components.

NB-3213.2 Gross Structural Discontinuity. Gross structural discontinuity is a geometric or material discontinuity which affects the stress or strain distribution through the entire wall thickness of the pressure retaining member.
Gross discontinuity type stresses are those portions of the actual stress distributions that produce net bending and membrane force resultants when integrated through the wall thickness. Examples of gross structural discontinuities are head-to-shell and flange-to-shell junctions, nozzles (NB-3331), and junctions between shells of different diameters or thicknesses.

As per Div-2

A gross structural discontinuity is a source of stress or strain intensification which affects a RELATIVELY LARGE PORTION of a structure and has a SIGNIFICANT EFFECT on the overall stress or strain pattern or on the structure as a whole.

This is against Local structural discontinuities that has an effect in the immediate vicinities eg fillets,

12. Local Structural Discontinuity – A source of stress or strain intensification which affects a relatively small volume of material and does not have a significant effect on the overall stress or strain pattern, or on the structure as a whole. Examples are small fillet radii, small attachments, and partial penetration welds.
(ASME VIII-2 para 5.12(12)