Question about TextureStage syntax

I have a question regarding the syntax of TextureStage.
For some reason I have not been able to find TextureStage in the API.

My question is “why do I need the 1 in the following line”:
setTexture(self.ts,model,1)

In my game I am simply putting a texture (roads) on a square and placing this square upon another model (grass).

self.ts=TextureStage("ts")
self.ts.setMode(TextureStage.MModulate) 
road=loader.loadTexture("models/road.PNG")
self.roadsquare.setTexture(self.ts,road,1)

I am guessing that the figure 1 has something to do with making it opague.

I could not find anything about the 1 in the manual (where I found TextureStage). Without it my square was just plain white; but out of sheer luck I tried putting the 1, and it worked.
Now I am curious, what does the 1 do?

Thanks.

The second argument is the priority level of the texture being set. A texture with a higher priority will be displayed on a model over a texture with a lower priority. You could set the priority to 2,3,4… depending on which texture you want to show in your game. This way, if your game sets a lower priority texture on a nodepath (through parenting or some other method) and you don’t want the texture to change, the lower priority texture wont show.