If you look at the bevels for GMAW and FCAW, they are less than for SMAW. SMAW rods are typically larger in diameter than are either GMAW or FCAW. In order to weld to the root you need more clearance,(SMAW) thus the more open the joint. So of you prepare an SMAW joint it will allow for the other processes because they are smaller and will not cause access problems.
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Dennis