premultiplied alpha? I would like to know in blender (internal) if premultiplied alpha is the same effect of "alpha clipping" in the alpha blend mode? 

I have found this :

"PREMULTIPLICATED ALPHA

The transparency information is stored in the alpha channel and also in the visible RGB channels, which are multiplied with a background color. The colors of semitransparent areas, such as smoothed edges, are tinted in the background color in proportion to their degree of transparency.

If white pixels have a value of 1 (figure) and black pixels a value of 0 (transparent background) and multiply them by the pixels of the RGB image, pixels multiplied by 1 will maintain their values ​​while pixels multiplied by 0 , Will become black. The problem comes when it is a value between 0 and 1, that is, a grayscale.

When composing these types of images on another background is when problems often occur. These are evidenced in the contours where, due to antialiasing or other effects, some of the pixels are gray, thus they are semitransparent. When composing our images, that semitransparency will show the black pixels of the background. When in addition, the alpha image contains motion blur, the problem is usually much worse, since it increases the number of pixels affected by the premultiplication."

that is a theory ,but I would like to inherence in blender.
No, alpha clipping refers to the amount of clipping applied to the alpha channel in the viewport (that is, what gets transparent and what gets solid in a face with an RGBA texture).

"Premultiplied alpha" refers to one of the two possible alpha modes:


Associated or Premultiplied
Unassociated or "straight" in Blender.


These alpha "modes" define which alpha over formula has to be used for blending the plate over a background.

When alpha is "associated", the alpha-over operation will simply add the foreground plate to the background previously scaled by the foreground's inverted alpha. 

When alpha is "unassociated" (straight), the foreground has to be pre multiplied by its alpha channel before the aforementioned operation, because the pixels with alpha 0 have RGB information that otherwise would be added to the background (and in the case of an unassociated alpha, it's not desired).

The term "pre multiplied" refers in the first case to the fact that the foreground doesn't have to be multiplied by alpha by the over operation, so it's considered as if it was already "pre multiplied". 

For that reason, it's adivisable to avoid the term "pre multiplied" becuase it suggests that there was a multiplication (which actually didn't happen). Alpha is "associated" as if it was already "pre multiplied".

Associated alpha allows more nuanced compositing effects than unassociated, as it allows to composite pixels that are both transparent and luminous, therefore is usually the recommended format for VFX (and it's the format that renderers like Blender's internal and cycles produce).