My take:

it means a certain set of standards set for the part by the US military contract. Made by the lowest bidder as cheap as possible


That being said, the biggest issue I have found with ARs:

1. Faulty Magazines - replace
2. Improper Lubrication - depends on environment. I found in AZ slip 200 EWL is best but colder environments may vary

Then the mechanical:

3. Incorrect ejector tension - causes FTE double feeds. Replace with an aftermarket spring and or oring if needed

4. Incorrect gas tube length and size - a lot of budget consumer ones and home builds experience this and are overgassed you can determine right away if it is not ejecting from about 2:30-4:45 and can be corrected with number 5, fewer are under gassed and require gas block port realignment

5. Incorrect spring buffer combo - replace with combo that is correct to provide proper ejection pattern. This is my most common fix other than the ejector spring.

6. Incorrect chamber size - rare but can be reamed usually experience failures to eject with lacquered steel case ammo where the case is stuck in the chamber and needs to be forced out .

That is just the ones I have gone over for the many people who have asked me to look at their AR buys and builds over the past 20 years.

The properly built AR is a pretty hardy weapon system no matter what myths the AK crowd likes to spread.:

S.W.A.T. MAGAZINE – FILTHY 14 : Slip 2000 Official Blog