ShaderLab: SubShader Tags { Queue }

http://docs.unity3d.com/Manual/SL-SubshaderTags.html

Rendering Order - Queue tag

• Background - this render queue is rendered before any others. You’d typically use this for things that really need to be in the background.
• Geometry (default) - this is used for most objects. Opaque geometry uses this queue.
• AlphaTest - alpha tested geometry uses this queue. It’s a separate queue from Geometry one since it’s more efficient to render alpha-tested objects after all solid ones are drawn.
• Transparent - this render queue is rendered after Geometry and AlphaTest, in back-to-front order. Anything alpha-blended (i.e. shaders that don’t write to depth buffer) should go here (glass, particle effects).
• Overlay - this render queue is meant for overlay effects. Anything rendered last should go here (e.g. lens flares).

For special uses in-between queues can be used. Internally each queue is represented by integer index; Background is 1000, Geometry is 2000, AlphaTest is 2450, Transparent is 3000 and Overlay is 4000. If a shader uses a queue like this:

Tags { "Queue" = "Geometry+1" }