What does #pragma multi_compile_particles do?

I’ve noticed this in pragma in Unity’s default shaders for particles but can’t find any documentation as to what it does.

	#pragma vertex vert
	#pragma fragment frag
	#pragma fragmentoption ARB_precision_hint_fastest

	#pragma multi_compile_particles

	#include "UnityCG.cginc"

Can anyone shed some light on what this does?

multi_compile is for (As the name suggests) compiling multiple instances using a set of keywords, that you define. Looking at the compiled shader, the multi_compile_particles adds SOFTPARTICLES_ON and SOFTPARTICLES_OFF, so it’s just Unity’s own flavor of keywords for particles, containing that one.
Additionally, you can toggle the keyword in quality settings.