What if none of my Texture Packs have MASK MAPs?

[goodGraphicGuy]

Hi i'm watching the Video tutorials for creating your own Biomes and Spawners, and one takes about Texture spawners...ANd there's 3 slots, one for the texture, one for the Normal , and one for the Mask map. mmmm, mask map? Before using Gaia i rarely see any of those in any of my texture packages?  The most i see is main texture, normals, height/bump map, ..none of my packs( and i have plenty and they were pricey) have any masks. So what do i do then? Can i leave that field blank?

 

Here's a sample of what i find when i search in my texture sets. There's no mask. so what am i supposed to do?