As a general rule the shade is related to the welding amperage in use.
At a DIY level of up to 200 amps, shade 11 is generally adequate.
When Gas Metal Arc Welding and Gas Tungsten Arc Welding of aluminium above 175 amps, we would use up to shade 13.
When Gas Metal Arc Welding using carbon dioxide (CO2) as the shielding gas we need to go to a higher shade as the welding amperage increases. For example: at around 150amps plus, use shade 13.
Lincoln Electric has a good online guide Welding Helmets and Eye Protection that covers welding and plasma processes and suggested shade lens against a given welding process and amperage.