Difference between float4 _LightColor0 and float4 _WorldSpaceLightPos0 ?

Hi, I have the following code in my shader:

Code (CSharp):

1.             float4 _LightColor0;
2.             //float4 _WorldSpaceLightPos0;
3.  

It works this way. As soon as I comment out

 _LightColor0;

the Shader won't compile anymore (it gives me "undeclared identifier _LightColor0"). I can use

_WorldSpaceLightPos0;

without declaring it first. Even more, as soon as I uncomment

//float4 _WorldSpaceLightPos0;

in the above, I get "redefinition of

_WorldSpaceLightPos0

".

What is the difference between these two global Unity Variables? Why do I have to declare one of them before using it while I am not allowed to declare the other one before using it?

For completeness, here is the complete shader I am using

Code (CSharp):

1. Shader "Unlit/Displacement/3. Specular"
2. {
3.     Properties
4.     {
5.         _MainTex ("Texture", 2D) = "white" {}
6.         _UOffset("U Offset", float) = 0
7.         _UMultiplicator("U Multiplikator", float) = 0.1
8.         _NoiseAmplitude("Noise Amplitude", float) = 1
9.         _WaterColor("Water Color", Color) = (0.2, 0.2, 0.35, 1)
10.         _Shininess("Shininess", float) = 1
11.     }
12.     SubShader
13.     {
14.         Tags { "RenderType"="Opaque" }
15.         LOD 100
16.  
17.         Pass
18.         {
19.             Tags {"LightMode" = "ForwardBase"}
20.             CGPROGRAM
21.             #pragma vertex vert
22.             #pragma fragment frag
23.             // make fog work
24.             #pragma multi_compile_fog
25.  
26.             #include "UnityCG.cginc"
27.  
28.             struct appdata
29.             {
30.                 float4 vertex : POSITION;
31.                 float2 uv : TEXCOORD0;
32.                 float3 normal : NORMAL;
33.             };
34.  
35.             struct v2f
36.             {
37.                 float2 uv : TEXCOORD0;
38.                 UNITY_FOG_COORDS(1)
39.                 float4 vertex : SV_POSITION;
40.                 float4 col : COLOR;
41.             };
42.            
43.             float4 _LightColor0;
44.             //float4 _WorldSpaceLightPos0;
45.  
46.             sampler2D _MainTex;
47.             float4 _MainTex_ST;
48.             float _UMultiplicator;
49.             float _UOffset;
50.             float _NoiseAmplitude;
51.             float4 _WaterColor;
52.             float Shininess;
53.  
54.             v2f vert (appdata v)
55.             {
56.                 v2f o;
57.                 //noise addieren
58.                 fixed4 noise = tex2Dlod(_MainTex, float4(v.uv.x*_UMultiplicator, v.uv.y, 0, 0));
59.                 v.vertex.xyz += noise.rgb;
60.                
61.                 float3 normalDirection = UnityObjectToWorldNormal(v.normal);
62.                 //directional lighting
63.                 float atten = 1.0;
64.                 float3 lightDirection = normalize(_WorldSpaceLightPos0.xyz);
65.                 float3 diffuseReflection = atten * _LightColor0.xyz * max(0.0, dot(normalDirection, lightDirection));
66.                 //add ambient lighting
67.                 float3 lightFinal = diffuseReflection + UNITY_LIGHTMODEL_AMBIENT.xyz;
68.                
69.                 o.col = float4(lightFinal * _WaterColor, 1.0);
70.                 //normales positionen veraendern
71.                 o.vertex = UnityObjectToClipPos(v.vertex);
72.                 o.uv = TRANSFORM_TEX(v.uv, _MainTex);
73.                 UNITY_TRANSFER_FOG(o,o.vertex);
74.                 return o;
75.             }
76.  
77.             fixed4 frag (v2f i) : SV_Target
78.             {
79.                 // sample the texture
80.                 fixed4 col = i.col;
81.                 // apply fog
82.                 UNITY_APPLY_FOG(i.fogCoord, col);
83.                 return col;
84.             }
85.             ENDCG
86.         }
87.     }
88. }
89.  

 

"_WorldSpaceLightPos0" included from "UnityShaderVariables.cginc" through "UnityCG.cginc".

 

Ah ok, after checking https://docs.unity3d.com/Manual/SL-UnityShaderVariables.html again I saw that

_LightPos0

is actually declared in

Lighting.cginc

, which I did not include.

Do I understand this right: If I want to use one of the globals here https://docs.unity3d.com/Manual/SL-UnityShaderVariables.html, I can either declare it myself or include the file in which it is declared?
Is there any runtime-cost associated with declaring globals but not using them (if I include

Lighthing.cginc

, but don't use much of it, will Unity still set all the globals in there every frame for the shader?)