Here you can put in any texture or shader black white, gray shaded and even colored to mask the whole material. you can use a black and white texture to cutout parts of the material (= Alpha).
You can use gradients or color filter to blend the materials with other vray materials or fade it into transparency. The material weight is good for many things. therefore we did not call it just Alpha.
But if you search an Alpha channel here it is:-)
Color Map – here an image texture or shader can be defined.
Mix Strength – defines the mixing proportion between the current material and other object materials.
Invert – This option simply inverts the Color Map, so transparent and solid areas are reversed. This works for both clipped images and images with built-in alpha channels
Additive Mode – hecking this option makes V-Ray Blend Material to behave like a multi-layered Shellac material. Note that this would often result in a physically incorrect material (e.g. a material that reflects more light than falls on it). It is not recommended to use this option unless you know what you are doing.
Do not use Invert button, with decal mapping or stacking, it could make unpredicted results on yours model texture look, so don`t use it with it.