Max aerobic speed

Interesting metric. I would think this would vary greatly due to head/tail wind, incline/decline, surface you are running on, altitude and other weather variables.  Unless you are constantly running indoors on a track I am not sure how useful this metric is. Then again you would not be able to track this indoors with no GPS!  Please enlighten me as I am not a runner but a cyclist. 

Yes, it can be converted to regular speed, or pace, so it suffers the same limitations.

But, you know that to improve your skills, you have to train at 100% MAS on short distance.

You can see some mas value for long distance in the link. For example a 10km can be run at 87-90% MAS.

https://www.kalenji.co.uk/advice/mashrmax-how-should-you-train-a_38621

So the goal is to improve you mas.

There are tests to evaluate your mas like cooper or vameval.

You use this metric while running on a outdoor track, or a flat road.

Wow, you learn something new everyday!  MAS is the slowest speed at which you reach VO_2max

https://fitness.edu.au/the-fitness-zone/article/training-at-maximum-aerobic-speed/#:~:text=The%20Fitness%20Zone.%201%205%20x%2015%20sec,MAS%20%2876m%29%20with%2015%20sec%20active%20recovery.%20

I can see how this is a simply and effective way to program workouts (without any extra gadgets HR monitor or Power pod).  I wonder if anyone has made a Garmin IQ app for this metric?? 

I guess you could also just map this all manually once you know your MAS but that is not your question (or map it to your Heart Rate or Power data).  It would also be cool if the Garmin could detect your MAS like it does threshold and VO2 MAX 

I made one. But it is not usable as limit in workout.

Wind isn't as big of a drag when running as it is for cycling, and most of the time when training off of a pace target you use a range similar to power or heart rate. There are a ton of calculators out there that tell you what paces to train at. For instance, the one I use has you enter in a recent race effort, (or if it's been over a couple of weeks I'll use the race predictor on the watch) along with a target fitness level. (So for instance I ran a 13.1 in 1:09:52, which according to the calculator is about 2:27:30 shape for a marathon, so I entered 2:25:00 for my target and then it gave me pace ranges to hit). As you accurately stated wind, grade, fatigue all factor into the equation, so especially for my easy days the range is quite large, (5:57-6:49) where as for a tempo run the range is pretty small 5:09-5:21 per mile, and then VO2 Max intervals are even smaller than that 4:56-5:04. Depending on weather you might make adjustments, (for me in the summer my coach and I opt for heart rate training as hitting those pace targets aren't realistic when it's 85 degrees outside, but there are also calculators to help you adjust your pace targets).