Version: Unity 6.1 Alpha (6000.1)
Language : English
Branch based on platform or graphics API
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Branch based on shader model

SHADER_TARGET is defined to a numeric value that matches the ShaderA program that runs on the GPU. More info
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target compilation model (that is, matching #pragma target directive). For example, SHADER_TARGET is 30 when compiling into Shader model 3.0. You can use it in Shader code to do conditional checks. For example:

#if SHADER_TARGET < 30
    // less than Shader model 3.0:
    // very limited Shader capabilities, do some approximation
#else
    // decent capabilities, do a better thing
#endif

Additional resources

Branch based on platform or graphics API
Branch based on platform features