Automatic Sorting of Translucent Surfaces
This method makes sure that all translucent surfaces are drawn in the correct order from the point of view outward. Half-Life does not guarantee this.
You also have the option to sort textures in studio models by making the model draw opaque textures first and textures with the additive flag last. This behaviour is enabled by setting the r_studio_sort_textures console variable to 1 (it is 0 by default).
For brushes, translucent surface sorting can be turned off by setting gl_nosort to 1.