Positional welding (other than flat) often relies on arc force and surface tension effect; therefore the welding position may affect the mechanical properties of the weld and the likelihood of defects.
this is often referred to as the 'self-adjusting arc'. Metal Inert Gas (MIG) welding is a 'flat' arc process (constant) voltage. Also known as Metal Active Gas (MAG); CO2; Metal-arc Gas Shielded, flux core and GMAW (US).