gfxStarfield=CreateImage(32,32,10)

gfxStarfield=CreateImage(32,32,10) 

; loop through each frame of the graphic we just made 
For t = 0 To 9 
; Set the drawing buffer to the graphic frame so we can write on it 
SetBuffer ImageBuffer(gfxStarfield,t) 
; put 50 stars in the frame at random locations 
For y = 1 To 50 
Plot Rnd(32),Rnd(32) 
Next 
Next 

this is a part of a sample code.

And my question is:
Why I can't close the
SetBuffer ImageBuffer(gfxStarfield,t)
End SetBuffer

I know that its not build for that , but how does blitz3d knows that he is done the the
SetBuffer ImageBuffer(gfxStarfield,t) ?

Or must I use it like this:
SetBuffer ImageBuffer(gfxStarfield,t)
;make the picture
SetBuffer BackBuffer() ;close it with this
;setbuffer too the default

Thanks