请教一个 GLSL 的问题

#version 450
layout(location = 0) in vec2 v_Uv;

layout(location = 0) out vec4 o_Target;

layout(set = 1, binding = 0) uniform CustomMaterial {
    vec4 Color;
};

layout(set = 1, binding = 1) uniform texture2D CustomMaterial_texture;
layout(set = 1, binding = 2) uniform sampler CustomMaterial_sampler;


void main() {
    o_Target = Color * texture(sampler2D(CustomMaterial_texture,CustomMaterial_sampler), v_Uv);
}

#version 450

layout(location = 0) in vec3 Vertex_Position;
layout(location = 1) in vec3 Vertex_Normal;
layout(location = 2) in vec2 Vertex_Uv;

layout(location = 0) out vec2 v_Uv;

layout(set = 0, binding = 0) uniform CameraViewProj {
    mat4 ViewProj;
    mat4 View;
    mat4 InverseView;
    mat4 Projection;
    vec3 WorldPosition;
    float width;
    float height;
};

layout(set = 2, binding = 0) uniform Mesh {
    mat4 Model;
    mat4 InverseTransposeModel;
    uint flags;
};

void main() {
    v_Uv = Vertex_Uv;
    gl_Position = ViewProj * Model * vec4(Vertex_Position, 1.0);
}

我想问一下这两个着色器里面 uniform 的 layout 里的 set 修饰符是什么意思，我知道后面的 binding 是 uniform 缓冲对象的绑定点，那顶点和片段着色器 uniform 前面的 set 代表什么呢，我翻阅 khronos 的文档也没查到这个修饰符。