Since these are so long, it would probably be better to find a way to share these RNA enum item arrays.

They can be moved to the top of rna_modifier.c and added to RNA_enum_types.h, then they should be usable here.

We don't do that elsewhere, but since these descriptions are a bit more complicated than usual, and more likely to change, and because the list is longer, I think it makes sense to do it here.

You can use nullptr instead of explicitly using IFACE* here since the RNA property names are the same as this anyway.

New line after function missing.

Casting to CustomDataType here is not correct, the cast should be to the type of the enum.

No need to move this line down, just use the same order used for other nodes:

ntype.geometry_node_execute = blender::nodes::geo_node_attribute_vector_math_exec;
ntype.draw_buttons = geo_node_attribute_vector_math_layout;
node_type_update(&ntype, blender::nodes::geo_node_attribute_vector_math_update);
node_type_init(&ntype, geo_node_attribute_vector_math_init);
node_type_storage(
    &ntype, "NodeAttributeVectorMath", node_free_standard_storage, node_copy_standard_storage);

Space after /*, period at the end of the comment.

I don't think there's any need for padding here, I may be wrong though.

Add newline

You'll notice that the names of rna enums exposed outside of a function's scope are longer, so they give more context.

So maybe rna_enum_subdivision_smooth_items, or something like that. Likewise for the other enum.

Add a blank line between the two enums

You probably meant to delete "static" too

Copy the descriptions for these two properties from the modifier RNA properties.

Extra newline

Add newline between functions

I don't think CustomDataType is the correct enum here...

The two functions that use these types just get int arguments, so best to keep it at that.

This comment isn't correct, eSubsurfUVSmooth is the right one I think, then eSubsurfBoundarySmooth for the next.

Add new line to separate these from the collection enum