The numbers refer to the alloy -
6000 series alloys are aluminium + magnesium + silicon
7000 series alloys are aluminium + zinc + magnesium
Both types can be welded and heat-treated. 7000 alloys are harder and more resistant to fatigue than 6000 series and are usually lighter for a given amount. This is why 7000 series alloys are used predominantly in the construction of aircraft frames.
However, 6000 series alloys are more maleable (i.e. they can be formed into shape more easily) and have a higher corrosion resistance than 7000 series.
For this reason, cheaper mass-produced bike frames are usually 7000 alloys, but frames which have butted or hydroformed tubing profiles are 6000 series.
You may also see the suffix T4 or T6 - this refers to the heat-treatment process that has been applied to the frame after welding to relieve unwanted stresses in the frame caused by the welding process.
There's an interesting article on aluminium alloys HERE.
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