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Showing content from https://docs.unity3d.com/Manual/sprite/../SL-VertexProgramInputs.html below:

Manual: Input vertex data into a shader

Use 16-bit precision in shaders

For Cg/HLSL vertex programs, the MeshThe main graphics primitive of Unity. Meshes make up a large part of your 3D worlds. Unity supports triangulated or Quadrangulated polygon meshes. Nurbs, Nurms, Subdiv surfaces must be converted to polygons. More info
See in Glossary vertex data is passed as inputs to the vertex shader function. Each input needs to have a semantic specified for it: for example, POSITION input is the vertex position, and NORMAL is the vertex normal.

Often, vertex data inputs are declared in a structure, instead of listing them one by one.

Several commonly used vertex structures are defined in UnityCG.cginc include file, and in most cases it’s enough just to use those. The structures are:

• appdata_base: position, normal and one texture coordinate.
• appdata_tan: position, tangent, normal and one texture coordinate.
• appdata_full: position, tangent, normal, four texture coordinates and color.
• appdata_img: vertex shaderA program that runs on the GPU. More info
  See in Glossary input with position and one texture coordinate.

This shader colors the mesh based on its normals, and uses appdata_base as vertex program input:

Shader "VertexInputSimple" {
    SubShader {
        Pass {
            HLSLPROGRAM
            #pragma vertex vert
            #pragma fragment frag
            #include "UnityCG.cginc"
         
            struct v2f {
                float4 pos : SV_POSITION;
                fixed4 color : COLOR;
            };
            
            v2f vert (appdata_base v)
            {
                v2f o;
                o.pos = UnityObjectToClipPos(v.vertex);
                o.color.xyz = v.normal * 0.5 + 0.5;
                o.color.w = 1.0;
                return o;
            }

            fixed4 frag (v2f i) : SV_Target { return i.color; }
            ENDHLSL
        }
    } 
}

To access different vertex data, you need to declare the vertex structure yourself, or add input parameters to the vertex shader. Vertex data is identified by Cg/HLSL semantics, and must be from the following list:

• POSITION is the vertex position, typically a float3 or float4.
• NORMAL is the vertex normal, typically a float3.
• TEXCOORD0 is the first UV coordinate, typically float2, float3 or float4.
• TEXCOORD1, TEXCOORD2 and TEXCOORD3 are the 2nd, 3rd and 4th UV coordinates, respectively.
• TANGENT is the tangent vector (used for normal mapping), typically a float4.
• COLOR is the per-vertex color, typically a float4.

When the mesh data contains fewer components than are needed by the vertex shader input, the rest are filled with zeroes, except for the .w component which defaults to 1. For example, mesh texture coordinates are often 2D vectors with just x and y components. If a vertex shader declares a float4 input with TEXCOORD0 semantic, the value received by the vertex shaderA program that runs on each vertex of a 3D model when the model is being rendered. More info
See in Glossary will contain (x,y,0,1).

For best cross platform support, label vertex outputs and fragment inputs as TEXCOORDn semantics.

Unity also supports the following:

• VPOS- the position of the pixelThe smallest unit in a computer image. Pixel size depends on your screen resolution. Pixel lighting is calculated at every screen pixel. More info
  See in Glossary being rendered. The shader must have a #pragma target 3.0 compilation directive, and should output the clip space position as a separate “out” variable. For maximum portability use the UNITY_VPOS_TYPE type for it, which is float4 on most platforms.
• VFACE - whether the rendered surface is facing the cameraA component which creates an image of a particular viewpoint in your scene. The output is either drawn to the screen or captured as a texture. More info
  See in Glossary, or facing away from the camera. The shader must have a #pragma target 3.0 compilation directive.
• SV_VertexID - the index of the vertex being processed. The shader must have a #pragma target 3.5 compilation directive.

For examples of using these techniques to visualize vertex data in the Built-in Render PipelineA series of operations that take the contents of a Scene, and displays them on a screen. Unity lets you choose from pre-built render pipelines, or write your own. More info
See in Glossary, see Visualizing vertex data.

Direct3D 11 groups all Shader variables into “constant buffers”. Most of Unity’s built-in variables are already grouped, but for variables in your own Shaders it might be more optimal to put them into separate constant buffers depending on expected frequency of updates.

Use CBUFFER_START(name) and CBUFFER_END macros for that:

CBUFFER_START(MyRarelyUpdatedVariables)
    float4 _SomeGlobalValue;
CBUFFER_END

If you use a GPU compute buffer or graphics buffer to set the value of the variables, make sure the buffer and the constant buffer have matching data layouts on all graphics APIs you build for. See Using constant buffers with GPU buffers for more information.

Note: You can’t add structs to constant buffers.

There are limits to how many interpolator variables can be used in total to pass the information from the vertex into the fragment shader. The limit depends on the platform and GPU, and the general guidelines are:

• Up to 8 interpolators: Direct3D 11 9.x level (Windows Phone) . Since the interpolator count is limited, but each interpolator can be a 4-component vector, some shaders pack things together to stay within limits. For example, you can pass two texture coordinates in one float4 variable (.xy for one coordinate, .zw for the second coordinate).
• Up to 10 interpolators: Shader model 3.0 (#pragma target 3.0).
• Up to 16 interpolators: OpenGL ES 3.0 (Android), Metal (iOS).
• Up to 32 interpolators: Direct3D 10 shader model 4.0 (#pragma target 4.0).

Regardless of your particular target hardware, it’s generally a good idea to use as few interpolators as possible for performance reasons.

• Built-in shader variables reference
• Customize how shaders contribute lightmap data in the Built-In Render Pipeline
• HLSL shader examples in the Built-In Render Pipeline
• Semantics in the Microsoft HLSL documentation

Use 16-bit precision in shaders

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