I'm not explaining it clearly.

A block-decorated struct describes the content of a binding. That's true for uniform-buffer and storage-buffer.
The last element of the struct of a storage buffer may be an unsized array.
(From memory, the

What I'm saying is that, with an extension, Vulkan can do unsized arrays of those block-decorated structs.
The use case is bindless-style programming, where only the binding entries accessed from the descriptor array need to have been bound.

Here's an example in GLSL for a compute shader:

#version 450

// This extension allows non-uniform indexing of the runtime-arrays of UBO and SSBO
// See SPIR-V capability RuntimeDescriptorArray
#extension GL_EXT_nonuniform_qualifier : require

layout(set=0,binding=0) uniform U {
  uint a;
  float data[10]; // sized, hence OpTypeArray
} myuni [];       // becomes OpTypeRuntimeArray

layout(set=0,binding=1) buffer B {
  uint a;
  float data[];   // unsized allowed.  Becomes OpTypeRuntimeArray
} mybuf [];       // becomes OpTypeRuntimeArray

void main() {
  uint i = myuni[0].a;
  mybuf[i].data[0] = myuni[i*i].data[0];
}

Compiling through "glslangValidator -V", we get this SPIR-V assembly:

               OpCapability Shader
               OpCapability RuntimeDescriptorArray
               OpExtension "SPV_EXT_descriptor_indexing"
          %1 = OpExtInstImport "GLSL.std.450"
               OpMemoryModel Logical GLSL450
               OpEntryPoint GLCompute %main "main"
               OpExecutionMode %main LocalSize 1 1 1
               OpSource GLSL 450
               OpSourceExtension "GL_EXT_nonuniform_qualifier"
               OpName %main "main"
               OpName %i "i"
               OpName %U "U"
               OpMemberName %U 0 "a"
               OpMemberName %U 1 "data"
               OpName %myuni "myuni"
               OpName %B "B"
               OpMemberName %B 0 "a"
               OpMemberName %B 1 "data"
               OpName %mybuf "mybuf"
               OpDecorate %_arr_float_uint_10 ArrayStride 16
               OpMemberDecorate %U 0 Offset 0
               OpMemberDecorate %U 1 Offset 16
               OpDecorate %U Block
               OpDecorate %myuni DescriptorSet 0
               OpDecorate %myuni Binding 0
               OpDecorate %_runtimearr_float ArrayStride 4
               OpMemberDecorate %B 0 Offset 0
               OpMemberDecorate %B 1 Offset 4
               OpDecorate %B BufferBlock
               OpDecorate %mybuf DescriptorSet 0
               OpDecorate %mybuf Binding 1
       %void = OpTypeVoid
          %3 = OpTypeFunction %void
       %uint = OpTypeInt 32 0
%_ptr_Function_uint = OpTypePointer Function %uint
      %float = OpTypeFloat 32
    %uint_10 = OpConstant %uint 10
%_arr_float_uint_10 = OpTypeArray %float %uint_10
          %U = OpTypeStruct %uint %_arr_float_uint_10
%_runtimearr_U = OpTypeRuntimeArray %U
%_ptr_Uniform__runtimearr_U = OpTypePointer Uniform %_runtimearr_U
      %myuni = OpVariable %_ptr_Uniform__runtimearr_U Uniform
        %int = OpTypeInt 32 1
      %int_0 = OpConstant %int 0
%_ptr_Uniform_uint = OpTypePointer Uniform %uint
%_runtimearr_float = OpTypeRuntimeArray %float
          %B = OpTypeStruct %uint %_runtimearr_float
%_runtimearr_B = OpTypeRuntimeArray %B
%_ptr_Uniform__runtimearr_B = OpTypePointer Uniform %_runtimearr_B
      %mybuf = OpVariable %_ptr_Uniform__runtimearr_B Uniform
      %int_1 = OpConstant %int 1
%_ptr_Uniform_float = OpTypePointer Uniform %float
       %main = OpFunction %void None %3
          %5 = OpLabel
          %i = OpVariable %_ptr_Function_uint Function
         %19 = OpAccessChain %_ptr_Uniform_uint %myuni %int_0 %int_0
         %20 = OpLoad %uint %19
               OpStore %i %20
         %26 = OpLoad %uint %i
         %28 = OpLoad %uint %i
         %29 = OpLoad %uint %i
         %30 = OpIMul %uint %28 %29
         %32 = OpAccessChain %_ptr_Uniform_float %myuni %30 %int_1 %int_0
         %33 = OpLoad %float %32
         %34 = OpAccessChain %_ptr_Uniform_float %mybuf %26 %int_1 %int_0
               OpStore %34 %33
               OpReturn
               OpFunctionEnd

Do we need [[block]] annotations? #1008

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