Render translucent textures. All the general rules for drawing translucent things apply. However, these modes have a major limitation: they only work with memory bitmaps or linear frame buffers (not with banked frame buffers). Don't even try, they do not check and your program will die horribly (or at least draw wrong things).
The man page POLYTYPE_PTEX_TRANS(3) is an alias of POLYTYPE_ATEX_TRANS(3).