Paint masks
Select terrain all surfaces that you want to paint mask for and then double-click on 
button.
By double-clicking on this icon, the script will disconnect any map currently connected to translucency
and make it ready to assign and paint a new map.
Assign a new map to “translucency”, don’t set a high resolution as it is only going to mask texture, usually 256×256 is more than enough.
WARNING: MAKE SURE TO SAVE YOUR SCENE RIGHT AFTER YOU CREATED A NEW MAP OR ELSE MAYA
WILL NOT SAVE THIS MAP AND IT CAN CAUSE YOUR TERRAIN TO GET RUINED
After you are done painting your mask, select a surface (only one) and then a texture handle and press the 
button.
Let’s say you have created a mask for multiple texture handles If you want to continue painting a mask just select terrain surfaces and at the end the texture handle that you want to edit and then single click on button, and you can continue editing that mask. (on tool setting make sure you are painting translucency attribute).