MIG welder question

This welder caught my attention being a 175A welder that comes with a spool gun and has favorable reviews:
I downloaded the manual and here's what it says about the spool gun.
Your MIG175 can operate a hand-held spool gun. This will allow you weld both Aluminum and Steel.
NOTE: for use on steel 18ga - 1/4"; aluminum 1/8" plate or thicker.
1. Turn the power off and remove plug from AC outlet
2. Change the internal switch from MIG to Spool Gun
3. Plug spool gun into outlet on the front panel of the unit.
4. Install 4" roll of steel or aluminum (4043) wire
NOTE: Spool gun with Mig will only weld aluminum plate 1/8" and thicker
Anyone here know why it isn't recommended for aluminum thinner than 1/8"? Anyone know why they specify 4043 wire? I'm just wondering why these limitations that other spool guns don't seem to have (that I know of).
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